Investing Activities - Sources of Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Investments, Debt and Equity Securities [Abstract]
Debt securities	$ 34.7	[1]	$ 31.7	[1]	$ 99.3	[1]	$ 92.7	[1]
Policy loans	0.7		0.7		2.2		2.3
Limited partnerships and other investments	0.3		0.8		1.6		1.8
Fair value investments	0.9		0.4		1.5		0.6
Total investment income	36.6		33.6		104.6		97.4
Less: Investment expenses	0.3		0.3		1.2		0.9
Net investment income	$ 36.3		$ 33.3		$ 103.4		$ 96.5

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