Investments - Mortgage Loans, Net (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 6,233	$ 5,470
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,247	5,132
Commercial mortgage loans | Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,223	5,058
Commercial mortgage loans | Commercial mortgage loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	24	74
Residential mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 986	$ 338