Investments - Mortgage Loans (Details) - USD ($)	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Maximum loan to value ratio generally allowed	75.00%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	$ 4,030,100,000	$ 3,730,300,000
Collective valuation allowance for losses	(1,000,000)	(1,200,000)
Total net commercial mortgage loans	4,029,100,000	3,729,100,000
Impairments on mortgage loans	0
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,700,000	10,700,000
Total net commercial mortgage loans	4,700,000	10,700,000
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,025,400,000	3,719,600,000
Collective valuation allowance for losses	(1,000,000)	(1,200,000)
Total net commercial mortgage loans	$ 4,024,400,000	$ 3,718,400,000