Investments - Mortgage Loans (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 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		$ 5,003,000,000		$ 4,911,000,000
Collective valuation allowance for losses		(1,000,000)		(1,000,000)
Total net commercial mortgage loans		5,002,000,000		4,910,000,000
Impairment of Real Estate	$ 0	0	$ 0
Mortgage loans in process of foreclosure		0		0
30 days or less in arrears
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in arrears		0		0
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans		4,000,000		4,000,000
Total net commercial mortgage loans		4,000,000		4,000,000
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans		4,999,000,000		4,907,000,000
Collective valuation allowance for losses		(1,000,000)		(1,000,000)
Total net commercial mortgage loans		$ 4,998,000,000		$ 4,906,000,000