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CORPORATE BORROWINGS - Senior Unsecured Convertible Notes (Details) $ / shares in Units, $ in Millions							1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) item $ / shares	Sep. 25, 2018 USD ($) $ / shares	Sep. 14, 2018 USD ($) item $ / shares shares	Jul. 24, 2018 USD ($)	May 03, 2018 USD ($)	Feb. 28, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Mar. 31, 2018 $ / shares	Dec. 31, 2017 USD ($)
CORPORATE BORROWINGS
Discount	$ (66.2)						$ (66.2)	$ (66.2)		$ (66.2)			$ 26.8
Deferred charges	(107.6)						(107.6)	(107.6)		(107.6)			$ (103.7)
Derivative liability	144.5						144.5	144.5		144.5
Special dividend value			$ 162.9	$ 25.8	$ 26.0	$ 26.0
Value of stock repurchased and canceled								422.9		422.9
Interest expense								64.3	$ 60.8	188.2	$ 171.7
Price per share (in dollars per share) \| $ / shares												$ 5.19
Senior Unsecured Convertible Notes due 2024
CORPORATE BORROWINGS
Principal balance	600.0		600.0				600.0	600.0		600.0
Discount	(89.8)		(90.4)				(89.8)	(89.8)		(89.8)
Deferred charges	(12.4)		(12.5)				(12.4)	(12.4)		(12.4)
Derivative liability	144.5		90.4				144.5	144.5		144.5
Carrying value	$ 642.3		$ 587.5				$ 642.3	$ 642.3		$ 642.3
Stated interest rate (as a percent)	2.95%		2.95%				2.95%	2.95%		2.95%
Effective interest rate			5.98%
Interest expense							$ 1.5
If-converted value in excess of principal							$ 49.1
Price per share (in dollars per share) \| $ / shares	$ 20.50						$ 20.50	$ 20.50		$ 20.50
Conversion rate			52.7704
Conversion rate (in dollars per share) \| $ / shares			$ 18.95
Number of shares upon conversion								5,900,000		2,000,000
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%
Senior Unsecured Convertible Notes due 2024 \| Charges to Earnings
CORPORATE BORROWINGS
Discount	$ 0.6						$ 0.6	$ 0.6		$ 0.6
Deferred charges	0.1						0.1	0.1		0.1
Derivative liability	54.1						54.1	54.1		54.1
Carrying value	$ 54.8						$ 54.8	$ 54.8		$ 54.8
Senior Unsecured Convertible Notes due 2024 \| Class A and B common stock
CORPORATE BORROWINGS
Special dividend (in dollars per share) \| $ / shares		$ 1.55
Special dividend value		$ 160.5
Legal fees		$ 1.9
Senior Unsecured Convertible Notes due 2024 \| Class A common stock
CORPORATE BORROWINGS
Conversion rate (in dollars per share) \| $ / shares	$ 18.95						$ 18.95	$ 18.95		$ 18.95
Number of shares upon conversion	31,662,269									31,662,269
Senior Unsecured Convertible Notes due 2024 \| Class B common stock
CORPORATE BORROWINGS
Common stock repurchased and cancellation (in shares) \| shares			24,057,143
Value of stock repurchased and canceled			$ 421.0
Price per share (in dollars per share) \| $ / shares			$ 17.50
Number of shares upon conversion \| item			5,666,000