Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Trading Securities, Realized Gain (Loss)	$ 76.9	$ 166.8	$ 95.7
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.0	(0.2)
Net realized investment gains, pre-tax	77.3	166.8	95.5
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(22.8)	(28.4)	(16.2)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.1
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(22.8)	(28.4)	(16.1)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	54.1	138.4	79.5
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	0.4	0.0	(0.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	54.5	138.4	79.4
Fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	1.9	5.8	(0.2)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	(0.2)
Net realized investment gains, pre-tax	1.9	5.8	(0.4)
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)			0.1
Foreign Currency Transaction Gain (Loss), Realized			0.0
Net realized investment gains, pre-tax			0.1
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	64.4	138.0	89.4
Foreign Currency Transaction Gain (Loss), Realized	0.4	0.0	0.0
Net realized investment gains, pre-tax	64.8	138.0	89.4
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	10.6	23.0	6.4
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0
Net realized investment gains, pre-tax	$ 10.6	$ 23.0	$ 6.4