Investments - Mortgage Loans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
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	$ 5,017,000,000		$ 5,017,000,000		$ 4,911,000,000
Collective valuation allowance for losses	(1,000,000)		(1,000,000)		(1,000,000)
Total net commercial mortgage loans	5,016,000,000		5,016,000,000		4,910,000,000
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	4,000,000		4,000,000		4,000,000
Total net commercial mortgage loans	4,000,000		4,000,000		4,000,000
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	5,013,000,000		5,013,000,000		4,907,000,000
Collective valuation allowance for losses	(1,000,000)		(1,000,000)		(1,000,000)
Total net commercial mortgage loans	$ 5,012,000,000		$ 5,012,000,000		$ 4,906,000,000