Investments - Mortgage Loans, Net (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 6,139	$ 6,233
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,133	5,247
Commercial mortgage loans | Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	5,109	5,223
Commercial mortgage loans | Commercial mortgage loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	24	24
Residential mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 1,006	$ 986