Investments - Mortgage Loans, Loan to Value Ratio (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 11,333	$ 10,631
Commercial mortgage loans
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	7,779	7,297
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	7,693	7,217
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Less than 50%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	2,060	1,883
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 50% to 60%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	1,986	1,988
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 61% to 70%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	2,856	2,394
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 71% to 80%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	702	898
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 81% to 100%
Financing Receivable, Recorded Investment [Line Items]
Mortgage loans, net of allowances	$ 89	$ 54