Investing Activities - Sources of Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Investments, Debt and Equity Securities [Abstract]
Debt securities	$ 39.8	[1]	$ 32.7	$ 77.6	$ 64.6	[1]
Limited partnerships and other investments	1.1		0.6	2.3	1.3
Policy loans	0.8		0.8	1.6	1.5
Fair value investments	0.6		0.3	1.4	0.6
Total investment income	42.3		34.4	82.9	68.0
Less: Investment expenses	0.3		0.3	0.9	0.9
Net investment income	$ 42.0		$ 34.1	$ 82.0	$ 67.1

[1] Includes net investment income on short-term investments.