Investments - Mortgage Loans, Debt Service Coverage Ratio (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 6,445	$ 5,470
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,503	5,132
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,503	5,058
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	4,992	4,378
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	233	353
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	$ 278	$ 327