Investments (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, various counterparties	$ 4,400.0	$ 2,700.0
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, U.S. state regulatory authorities	1,700.0	842.6
Reverse repurchase agreements	$ 30.0	$ 78.7
Minimum percentage of collateral required	102.00%