Investments Investments (Restricted Assets and Special Deposits) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral	$ 16,254	$ 16,402
Securities Sold under Agreements to Repurchase
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral	8,400	7,606
Cash Collateral For Loaned Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral	4,354	4,333
Separate Account Liabilities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral	3,064	3,462
Policyholders' Account Balances
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral	$ 436	$ 1,001