INVESTMENTS (SCHEDULE OF INVESTMENTS WITH UNREALIZED LOSSES CLASSIFIED BY CONTRACTUAL MATURITY DATE) (Details) $ in Millions	Dec. 31, 2016 USD ($)
Amortized cost
Due in one year or less	$ 35.7
Due after one year through five years	150.4
Due after five years through ten years	389.3
Due after ten years	2,262.3
Subtotal	2,837.7
Structured securities	1,857.2
Total	4,694.9
Estimated fair value
Due in one year or less	35.0
Due after one year through five years	145.9
Due after five years through ten years	370.9
Due after ten years	2,136.9
Subtotal	2,688.7
Structured securities	1,811.9
Total	$ 4,500.6