INVESTMENTS (SCHEDULE OF INVESTMENTS CLASSIFIED BY CONTRACTUAL MATURITY DATE) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less	$ 354.7
Due after one year through five years	2,243.8
Due after five years through ten years	1,549.1
Due after ten years	10,395.0
Subtotal	14,542.6
Structured securities	5,260.5
Amortized cost	19,803.1	$ 18,947.0
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Due in one year or less	359.8
Due after one year through five years	2,399.5
Due after five years through ten years	1,620.8
Due after ten years	11,320.9
Subtotal	15,701.0
Structured securities	5,395.2
Estimated fair value	$ 21,096.2	$ 19,882.9