Income From Investments - Net Realized Gains on Sales of Investments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Realized Investment Gains									$ 38.0	$ 55.3	$ 41.7
Realized Investment Losses									(5.1)	(2.9)	(2.5)
Gain on Sale of Subsidiary									0.0	0.0	1.6
Net Gains (Losses) on Trading Securities									0.2	(0.3)	(0.1)
Net Realized Gains (Losses) on Sales of Investments	$ 9.1	$ 11.6	$ 5.6	$ 6.8	$ 9.4	$ 5.3	$ 34.0	$ 3.4	33.1	52.1	39.1
Fixed Maturities [Member]
Net Investment Income [Line Items]
Gains on Sales									17.0	16.1	7.0
Losses on Sales									(4.6)	(1.1)	(0.2)
Equity Securities [Member]
Net Investment Income [Line Items]
Gains on Sales									19.9	39.2	33.1
Losses on Sales									(0.3)	(1.6)	(2.0)
Real Estate Investment [Member]
Net Investment Income [Line Items]
Realized Investment Gains									1.1	0.0	0.0
Realized Investment Losses									0.0	(0.2)	(0.2)
Other Investment [Member]
Net Investment Income [Line Items]
Realized Investment Losses									$ (0.2)	$ 0.0	$ (0.1)