Investments - Mortgage Loans, Net (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 9,371	$ 6,233
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	6,670	5,247
Commercial mortgage loans | Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	6,596	5,223
Commercial mortgage loans | Commercial mortgage loans under development
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	74	24
Residential mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 2,701	$ 986