Investments - Commercial Mortgage Loans Credit Quality (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,175	$ 2,886
Commercial Loan
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,175	$ 2,886
Avg. Debt-Service Coverage Ratio	2.69	2.83
Commercial Loan | Greater than 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 18	$ 0
Avg. Debt-Service Coverage Ratio	1.27	0.00
Commercial Loan | 65% - 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 265	$ 386
Avg. Debt-Service Coverage Ratio	1.95	2.16
Commercial Loan | Less than 65%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 2,892	$ 2,500
Avg. Debt-Service Coverage Ratio	2.76	2.95