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 |