Investments - Mortgage Loans, Loan to Value Ratio (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 9,371	$ 6,233
Commercial mortgage loans
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	6,670	5,247
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	6,596	5,223
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Less than 50%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	1,805	1,841
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 50% to 60%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	1,776	1,390
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 61% to 70%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	2,186	1,691
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 71% to 80%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	764	239
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 81% to 100%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 65	$ 62