Investments - Mortgage Loans, Debt Service Coverage Ratio (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 12,199	$ 10,631
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	8,117	7,297
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	8,022	7,217
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Greater than 1.20x
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	7,220	6,576
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 1.00x – 1.20x
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	394	474
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Less than 1.00x
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 408	$ 167