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CORPORATE BORROWINGS - Senior Unsecured Convertible Notes (Details)											1 Months Ended	4 Months Ended	12 Months Ended
	Oct. 24, 2019 USD ($) $ / shares	Aug. 02, 2019 USD ($) $ / shares	May 03, 2019 USD ($) $ / shares	Feb. 15, 2019 USD ($) $ / shares	Nov. 01, 2018 USD ($) $ / shares	Sep. 25, 2018 USD ($) $ / shares	Sep. 14, 2018 USD ($) item $ / shares shares	Dec. 19, 2016 USD ($) $ / shares	Sep. 19, 2016 USD ($) $ / shares	Mar. 21, 2016 USD ($) $ / shares	Jun. 20, 2016 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Jul. 14, 2018 USD ($)	Jun. 30, 2017 $ / shares
CORPORATE BORROWINGS
Principal balance							$ 600,000,000.0
Discount												$ (64,400,000)	$ (69,100,000)	$ (64,400,000)
Deferred charges												(104,300,000)	(88,800,000)	(104,300,000)
Derivative liability												$ 24,000,000.0	500,000	$ 24,000,000.0
Price per share (in dollars per share) \| $ / shares												$ 6.86		$ 6.86			$ 7.42
Value of stock repurchased and canceled														$ 412,600,000
Special dividend (in dollars per share) \| $ / shares	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.20			$ 1.55	$ 0.20	$ 0.20	$ 0.20
Special dividend value	$ 21,000,000.0	$ 21,300,000	$ 21,300,000	$ 21,300,000	$ 21,200,000		1.55	$ 162,900,000	$ 25,800,000	$ 26,000,000.0	$ 26,000,000.0
Interest expense													292,800,000	262,300,000	$ 231,600,000
Other income (expense) related to derivative assets and liabilities													(13,400,000)	108,100,000	$ 1,500,000
2.95% Senior Unsecured Convertible Notes due 2024
CORPORATE BORROWINGS
Principal balance							600,000,000.0					$ 600,000,000.0	600,000,000.0	600,000,000.0
Discount							(90,400,000)					(86,700,000)	(73,700,000)	(86,700,000)
Deferred charges							(12,500,000)					(13,000,000.0)	(11,200,000)	(13,000,000.0)
Derivative asset							10,700,000
Derivative liability							90,400,000					24,000,000.0	500,000	24,000,000.0
Carrying value							$ 587,500,000					$ 524,300,000	$ 515,600,000	$ 524,300,000
Stated interest rate (as a percent)							2.95%					2.95%	2.95%	2.95%
Price per share (in dollars per share) \| $ / shares													$ 7.24
Effective interest rate							5.98%
Deferred financing costs							$ 13,600,000
Interest expense							$ 32,600,000					$ 9,700,000	$ 32,600,000
Other income													23,500,000	$ 66,400,000
Other income (expense) related to derivative assets and liabilities													17,700,000	45,000,000.0
Change fair value of derivative													(23,500,000)	(66,400,000)
If-converted value in excess of principal													370,800,000
Minimum conversion price percentage causing a reset conversion price							120.00%
Number of days needed to cause a reset conversion price \| item							10
Maximum allowed percentage of outstanding fully-diluted share capital resulting a conversion price floor							30.00%
Maximum dividends allowed through the second anniversary of issuance \| $ / shares							$ 0.20
Maximum dividends allowed after the second anniversary of issuance \| $ / shares							$ 0.10
Threshold percentage of stock price trigger							150.00%
Debt Instrument, Convertible, Threshold Trading Days \| item							20
Debt Instrument, Convertible, Threshold Consecutive Trading Days \| item							30
Internal rate of return							15.00%
2.95% Senior Unsecured Convertible Notes due 2024 \| Additional Deferred Charges
CORPORATE BORROWINGS
Deferred charges												(1,100,000)		(1,100,000)
Carrying value												(1,100,000)		(1,100,000)
2.95% Senior Unsecured Convertible Notes due 2024 \| (Increase) decrease to Net Earnings (Loss)
CORPORATE BORROWINGS
Discount												3,700,000	13,000,000.0	3,700,000
Deferred charges												600,000	1,800,000	600,000
Derivative liability												(66,400,000)	(23,500,000)	(66,400,000)
Carrying value												$ (62,100,000)	$ (8,700,000)	$ (62,100,000)
2.95% Senior Unsecured Convertible Notes due 2024 \| Class A and B common stock
CORPORATE BORROWINGS
Legal fees						$ 2,600,000
Special dividend (in dollars per share) \| $ / shares						$ 1.55
Special dividend value						$ 160,500,000
2.95% Senior Unsecured Convertible Notes due 2024 \| Class A common stock
CORPORATE BORROWINGS
Principal balance																$ 1,000
Conversion rate							52.7704
Conversion rate (in dollars per share) \| $ / shares							$ 18.95						$ 18.95
Number of shares upon conversion													31,662,269
2.95% Senior Unsecured Convertible Notes due 2024 \| Class B common stock
CORPORATE BORROWINGS
Common stock repurchased and cancellation (in shares) \| shares							24,057,143
Price per share (in dollars per share) \| $ / shares							$ 17.50
Value of stock repurchased and canceled							$ 421,000,000.0
Legal fees							$ 2,600,000
Number of shares upon conversion \| item							5,666,000