Derivative Instruments and Other Financial Instruments Used For Hedging - Offsetting of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Financial Assets
Gross Amounts of Recognized Assets, Derivative assets	$ 1,828	$ 1,799
Gross Amounts Offset in Balance Sheet, Derivative assets	909	887
Net Amounts Presented in Balance Sheet, Derivative assets	919	912
Gross Amounts Not Offset In Balance Sheet, Financial Instruments, Derivative assets	60	113
Gross Amounts Not Offset in Balance Sheet, Cash Collateral Pledged, Derivative assets	0	0
Net Amount, Derivative assets	859	799
Gross Amounts of Recognized Assets, Securities purchased under resale agreements	24	62
Net Amounts Presented in Balance Sheet, Securities purchased under resale agreements	24	62
Gross Amount Not Offset in Balance Sheet, Financial Instruments, Securities purchased under repurchase agreements	24	61
Net Amount, Securities purchased under repurchase agreements	0	1
Gross Amounts of Recognized Assets, Financial Assets	1,852	1,861
Gross Amounts Offset in Balance Sheet, Financial Assets	909	887
Net Amounts Presented in Balance Sheet, Financial Assets	943	974
Gross Amounts Not Offset In Balance Sheet, Financial Instruments, Financial Assets	84	174
Gross Amounts Not Offset in Balance Sheet, Cash Collateral Pledged, Financial Assets	0	0
Net Amount, Financial Assets	859	800
Financial Liabilities
Gross Amounts of Recognized Liabilities, Derivative liabilities	1,647	1,624
Gross Amounts Offset in Balance Sheet, Derivative liabilities	872	787
Net Amounts Presented in Balance Sheet, Derivative liabilities	775	837
Gross Amount Not Offset in Balance Sheet, Financial Instruments, Derivative liabilities	180	162
Gross Amount Not Offset in Balance Sheet, Cash Collateral Pledged, Derivative liabilities	0	0
Net Amount, Derivative liabilities	595	675
Gross Amounts of Recognized Liabilities, Securities sold under repurchase agreements	36	68
Net Amounts Presented in Balance Sheet, Securities sold under repurchase agreements	36	68
Gross Amount Not Offset in Balance Sheet, Financial Instruments, Securities sold under repurchase agreements	36	68
Net Amount, Securities sold under repurchase agreements	0	0
Gross Amounts of Recognized Liabilities, Financial Liabilities	1,683	1,692
Gross Amounts Offset in Balance Sheet, Financial Liabilities	872	787
Net Amounts Presented in Balance Sheet, Financial Liabilities	811	905
Gross Amount Not Offset in Balance Sheet, Financial Instruments, Financial Liabilities	216	230
Gross Amount Not Offset in Balance Sheet, Cash Collateral Pledged, Financial Liabilities	0	0
Net Amount, Financial Liabilities	$ 595	$ 675