Investing Activities - Sources of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Investment Income [Line Items]
Investment income	$ 50.4	$ 42.3	$ 99.8	$ 82.9
Less: Investment expenses	2.1	0.3	4.4	0.9
Net investment income	48.3	42.0	95.4	82.0
Debt securities
Net Investment Income [Line Items]
Investment income	49.2	39.7	96.5	77.3
Equity securities
Net Investment Income [Line Items]
Investment income	0.5	0.2	1.0	0.4
Limited partnerships and other investments
Net Investment Income [Line Items]
Investment income	0.4	1.0	0.9	2.2
Policy loans
Net Investment Income [Line Items]
Investment income	0.8	0.8	1.6	1.6
Fair value investments
Net Investment Income [Line Items]
Investment income	$ (0.5)	$ 0.6	$ (0.2)	$ 1.4