Investing Activities - Sources of Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]
Debt securities	$ 42.3	[1]	$ 34.7	[1]	$ 119.9	[1]	$ 99.3	[1]
Limited partnerships and other investments	0.8		0.3		3.1		1.6
Policy loans	0.8		0.7		2.4		2.2
Fair value investments	0.5		0.9		1.9		1.5
Total investment income	44.4		36.6		127.3		104.6
Less: Investment expenses	0.3		0.3		1.2		1.2
Net investment income	$ 44.1		$ 36.3		$ 126.1		$ 103.4

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