Investments - Commercial Mortgage Loans Credit Quality (Details) $ in Millions	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 1,960	$ 2,872
Commercial Loan
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 1,960	$ 2,872
Avg. Debt-Service Coverage Ratio	2.55	2.65
Commercial Loan | Greater than 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 5	$ 5
Avg. Debt-Service Coverage Ratio	1.28	1.26
Commercial Loan | 65% - 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 151	$ 125
Avg. Debt-Service Coverage Ratio	1.78	1.88
Commercial Loan | Less than 65%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 1,804	$ 2,742
Avg. Debt-Service Coverage Ratio	2.62	2.69