Investments - Mortgage Loans (Details) - USD ($)	12 Months Ended
	Dec. 31, 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]
Commercial mortgage loans	$ 4,255,600,000	$ 3,730,300,000
Collective valuation allowance	(1,100,000)	(1,200,000)
Total net commercial mortgage loans	4,254,500,000	3,729,100,000
Impairments on mortgage loans	0	0
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage loans	4,600,000	10,700,000
Total net commercial mortgage loans	4,600,000	10,700,000
Not Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage loans	4,251,000,000	3,719,600,000
Collective valuation allowance	(1,100,000)	(1,200,000)
Total net commercial mortgage loans	$ 4,249,900,000	$ 3,718,400,000