Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments in and Advances to Affiliates [Line Items]
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates	$ 0.0	$ (39.2)	$ 81.2
Income tax benefit (expense)	0.0	2.9	(5.9)
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(36.3)	75.3
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(34.9)	75.3
Net Realized and Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	1.5	190.2	60.5
Total investment gains recorded during the period, after-tax	1.5	155.3	135.8
Symetra
Investments in and Advances to Affiliates [Line Items]
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0	(34.9)	75.3
Reversal of accumulated other comprehensive income related to change in accounting for the investment in Symetra	$ 0.0	$ 1.4	$ 0.0