Investments - Mortgage Loans, Debt Service Coverage Ratio (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 10,631	$ 6,233
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	7,297	5,247
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	7,217	5,223
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	6,576	4,742
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	474	297
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	$ 167	$ 184