Investments - Investments - Commercial Mortgage Loans Credit Quality (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,559	$ 3,175
Commercial Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,559	$ 3,175
Avg. Debt-Service Coverage Ratio	2.50	2.69
Commercial Loan [Member] | Greater than 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 0	$ 18
Avg. Debt-Service Coverage Ratio	0.00	1.27
Commercial Loan [Member] | 65% - 80%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 406	$ 265
Avg. Debt-Service Coverage Ratio	1.68	1.95
Commercial Loan [Member] | Less than 65%
Financing Receivable, Recorded Investment [Line Items]
Carrying Value	$ 3,153	$ 2,892
Avg. Debt-Service Coverage Ratio	2.60	2.76