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ACCUMULATED OTHER COMPREHENSIVE INCOME - Change in AOCI by component (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Changes in accumulated other comprehensive income      
Balance at the beginning of the period $ 1,397.6 $ 2,112.4 $ 2,009.6
Other comprehensive income (loss): (31.6) (124.5) 128.1
Balance at the end of the period 1,214.2 1,397.6 2,112.4
Accumulated Other Comprehensive Income (Loss)      
Changes in accumulated other comprehensive income      
Balance at the beginning of the period 5.5 125.6 (2.5)
Other comprehensive income (loss) before reclassifications (32.1) (123.3)  
Amounts reclassified from accumulated other comprehensive income 0.5 (1.2)  
Other comprehensive income (loss): (31.6) (124.5) 128.1
Balance at the end of the period (26.1) 5.5 125.6
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   (0.6)  
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   5.0  
Foreign Currency      
Changes in accumulated other comprehensive income      
Balance at the beginning of the period 7.2 129.9  
Other comprehensive income (loss) before reclassifications (16.5) (127.7) 131.7
Amounts reclassified from accumulated other comprehensive income 0.5 1.0  
Other comprehensive income (loss):   (126.7)  
Balance at the end of the period (8.8) 7.2 129.9
Foreign Currency | Accounting Standards Update 2018-02 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   4.0  
Pension and Other Benefits (recorded in G&A : Other)      
Changes in accumulated other comprehensive income      
Balance at the beginning of the period (1.8) (6.6)  
Other comprehensive income (loss) before reclassifications (15.5) 4.2  
Other comprehensive income (loss):   4.2  
Balance at the end of the period (17.3) (1.8) (6.6)
Pension and Other Benefits (recorded in G&A : Other) | Accounting Standards Update 2018-02 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   0.6  
Unrealized Gains from Marketable Securities (Recorded in Investment income)      
Changes in accumulated other comprehensive income      
Balance at the beginning of the period   0.6  
Other comprehensive income (loss) before reclassifications     0.7
Balance at the end of the period     0.6
Unrealized Gains from Marketable Securities (Recorded in Investment income) | Accounting Standards Update 2016-01 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   (0.6)  
Unrealized Gain from Equity Method Investees' Cash Flow Hedge      
Changes in accumulated other comprehensive income      
Balance at the beginning of the period 0.1 1.7  
Other comprehensive income (loss) before reclassifications $ (0.1) 0.2  
Amounts reclassified from accumulated other comprehensive income   (2.2) (0.9)
Other comprehensive income (loss):   (2.0)  
Balance at the end of the period   0.1 $ 1.7
Unrealized Gain from Equity Method Investees' Cash Flow Hedge | Accounting Standards Update 2018-02 [Member]      
Changes in accumulated other comprehensive income      
Amounts reclassified from accumulated other comprehensive income   $ 0.4