Derivatives (Offsetting Derivative Financial Instruments) (Detail) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016
Derivative Fair Value Gross Amount Assets Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	$ 823		$ 1,057
Derivative, Collateral, Obligation to Return Cash	(341)		(374)
Derivative Fair Value Gross Amount Liabilities Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	602		666
Assets
Derivative Fair Value Gross Amount Assets Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	815	[1]	1,044	[2]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(213)		(374)
Derivative, Collateral, Obligation to Return Cash	(362)	[3]	(377)	[4]
Derivative, Fair Value, Amount Offset Against Collateral, Net	240		293
Liabilities
Derivative Fair Value Gross Amount Liabilities Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	602	[1]	665	[2]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(213)		(374)
Derivative Liabilities, Collateral, Right to Reclaim Cash	(155)	[3]	(125)	[4]
Derivative Fair Value Amount Offset Against Collateral Net	234		166
Derivative | Assets
Derivative Fair Value Gross Amount Assets Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	815	[1]	1,044	[2]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(213)		(374)
Derivative, Collateral, Obligation to Return Cash	(362)	[3]	(377)	[4]
Derivative, Fair Value, Amount Offset Against Collateral, Net	240		293
Derivative | Liabilities
Derivative Fair Value Gross Amount Liabilities Not Offset Against Collateral Net
Gross Amount Recognized in the Balance Sheet	602	[1]	665	[2]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(213)		(374)
Derivative Liabilities, Collateral, Right to Reclaim Cash	(155)	[3]	(125)	[4]
Derivative Fair Value Amount Offset Against Collateral Net	$ 234		$ 166

[1] (a) Amount does not include IRLCs because these instruments are not subject to master netting or similar arrangements.

[2] Amount does not include IRLCs because these instruments are not subject to master netting or similar arrangement s .

[3] (b) Amount of collateral received as an offset to asset positions or pledged as an offset to liability positions. Collateral values in excess of related d erivative amounts recognized in the Consolidated Balance Sheets were excluded from this table.

[4] Amount of collateral received as an offset to asset positions or pledged as an offset to liability positions. Collateral values in excess of related derivative amounts recognized in the Consolidated Balance Sheets were excluded from this table.