Investments - Mortgage Loans (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Maximum loan to value ratio generally allowed			75.00%
Commercial mortgage loans	$ 3,189,400,000		$ 3,189,400,000		$ 2,874,000,000
Collective valuation allowance	(1,500,000)		(1,500,000)		(1,300,000)
Total net commercial mortgage loans	3,187,900,000		3,187,900,000		2,872,700,000
Impairments on mortgage loans	$ 0	$ 0	$ 0	$ 0