AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2018
Net activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period		$ 430		$ (460)
Reclassification adjustment for net (gains) losses included in net income		1		7
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period		87		(7)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income		2		(1)
Net activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs		(1)		(1)
Total Other Comprehensive Activity
Pre-tax activity total		675		(594)
Total, Tax		(154)		134
Other comprehensive income (loss)	$ 521	521	$ (460)	(460)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		(112)		73
Other comprehensive income (loss), Net of Tax	521	521	(460)	(460)
Total Accumulated Other Comprehensive Income - Ending Balance	409	409	(389)	(389)
Accumulated Net Unrealized Investment Gain (Loss)
Pre-tax activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period		561		(594)
Reclassification adjustment for net losses (gains) included in net income		1		9
Net unrealized gains on available-for-sale debt securities		562		(585)
Tax effect for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period		(131)		134
Reclassification adjustment for net losses (gains) included in net income		0		(2)
Net unrealized gains on available-for-sale debt securities		(131)		132
Net activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period		430		(460)
Reclassification adjustment for net (gains) losses included in net income		1		7
Net unrealized gains on available-for-sale debt securities		431		(453)
Total Other Comprehensive Activity
Other comprehensive income (loss)		431		(453)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		(227)		135
Other comprehensive income (loss), Net of Tax		431		(453)
Total Accumulated Other Comprehensive Income - Ending Balance	204	204	(318)	(318)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Pre-tax activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period		110		(9)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income		2		(1)
Net unrealized gains on cash flow hedge derivatives		112		(10)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period		(23)		2
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income		0		0
Net unrealized gains on cash flow hedge derivatives		(23)		2
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period		87		(7)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income		2		(1)
Net unrealized gains on cash flow hedge derivatives		89		(8)
Total Other Comprehensive Activity
Other comprehensive income (loss)		89		(8)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		160		(11)
Other comprehensive income (loss), Net of Tax		89		(8)
Total Accumulated Other Comprehensive Income - Ending Balance	249	249	(19)	(19)
Amortization Of Defined Benefit Pension Items
Pre-tax activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs		1		1
Defined benefit plans, net		1		1
Tax effect for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs		0		0
Defined benefit plans, net		0		0
Net activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs		(1)		(1)
Defined benefit plans, net		1		1
Total Other Comprehensive Activity
Other comprehensive income (loss)		1		1
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		(45)		(53)
Other comprehensive income (loss), Net of Tax		1		1
Total Accumulated Other Comprehensive Income - Ending Balance	$ (44)	$ (44)	$ (52)	$ (52)