Derivative Financial Instruments Netting of Financial Instruments - Derivatives (Additional Information) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset	[1]	$ 802	$ 984
Derivative Asset, Collateral, Obligation to Return Cash, Offset		371	291
Derivative Liability	[2]	408	398
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		1,303	1,265
Trading Securities [Member]
Derivative [Line Items]
Derivative Asset		802	984
Trading Liabilities [Member]
Derivative [Line Items]
Derivative Liability		$ 408	$ 398

[1] At December 31, 2017, $802 million, net of $371 million offsetting cash collateral, is recognized in Trading assets and derivative instruments within the Company's Consolidated Balance Sheets. At December 31, 2016, $984 million, net of $291 million offsetting cash collateral, is recognized in Trading assets and derivative instruments within the Company's Consolidated Balance Sheets.

[2] At December 31, 2017, $408 million, net of $1.3 billion offsetting cash collateral, is recognized in Trading liabilities and derivative instruments within the Company's Consolidated Balance Sheets. At December 31, 2016, $398 million, net of $1.3 billion offsetting cash collateral, is recognized in Trading liabilities and derivative instruments within the Company's Consolidated Balance Sheets.