Investments - Mortgage Loans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2014 loan		Sep. 30, 2013	Sep. 30, 2014 loan		Sep. 30, 2013	Dec. 31, 2013 loan
Investments, Debt and Equity Securities [Abstract]
Maximum loan to value ratio generally allowed				75.00%
Commercial mortgage loans	$ 9,953.1	[1]		$ 9,953.1	[1]		$ 9,316.0	[1]
Collective valuation allowance	(3.4)			(3.4)			(3.8)
Total net commercial mortgage loans	9,949.7			9,949.7			9,312.2
Impairments on mortgage loans	0		0	0		0
Number of mortgage loans in foreclosure	0			0			0.0
Past Due Mortgage Loans, Carrying Value:
30 days or less past due	0			0			0
31 to 90 days past due	0			0			5.1
91 to 180 days past due	0			0			0
181 days or more past due	0			0			0
Total	$ 0			$ 0			$ 5.1

[1] Balances do not include collective valuation allowance for losses.