Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized investment losses on investments in unconsolidated affiliates	$ 0.0	$ 0.0	$ (39.2)
Income tax benefit	0.0	0.0	2.9
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	0.0	0.0	(36.3)
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	120.4	(24.7)	212.8
Total investment gains (losses) recorded during the period, after-tax	120.4	(24.7)	177.9
Symetra
Investments in and Advances to Affiliates [Line Items]
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	0.0	0.0	1.4
Net change in unrealized investment losses on investments in unconsolidated affiliates, after-tax	$ 0.0	$ 0.0	$ (34.9)