Investments - Mortgage Loans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
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	$ 4,868,500,000		$ 4,868,500,000		$ 4,255,600,000
Collective valuation allowance for losses	(900,000)		(900,000)		(1,100,000)
Total net commercial mortgage loans	4,867,600,000		4,867,600,000		4,254,500,000
Impairment of Real Estate	0	$ 0	0	$ 0
Mortgage loans in process of foreclosure	0		0		0
30 days or less in arrears
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in arrears	0		0		0
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,300,000		4,300,000		4,600,000
Total net commercial mortgage loans	4,300,000		4,300,000		4,600,000
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,864,200,000		4,864,200,000		4,251,000,000
Collective valuation allowance for losses	(900,000)		(900,000)		(1,100,000)
Total net commercial mortgage loans	$ 4,863,300,000		$ 4,863,300,000		$ 4,249,900,000