Investments (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, various counterparties	$ 2,700.0	$ 2,500.0
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, U.S. state regulatory authorities	842.6	664.6
Reverse repurchase agreements	$ 78.7	$ 26.2
Minimum percentage of collateral required	102.00%