Investments - Mortgage Loans, Loan to Value Ratio (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 10,631	$ 6,233
Commercial mortgage loans
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	7,297	5,247
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	7,217	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	1,883	1,798
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 50% to 60%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	1,988	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	2,394	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	898	282
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 81% to 100%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 54	$ 62