AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pre-tax activity for accumulated net unrealized gain (loss) on available-for-sale securities
Reclassification adjustment for net losses (gains) included in net income		$ 1
Pre-tax activity for net unrealized gain (loss) on cash flow hedge derivatives
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	$ (32)	2
Net activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during the period	882	430
Reclassification adjustment for net (gains) losses included in net income		1
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during the period	427	87
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	25	(2)
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	1,665	675
Total, Tax	(380)	(154)
Other comprehensive income, net of tax	1,285	521
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	1,192	(112)
Other comprehensive income (loss), net of tax	1,285	521
Total Accumulated Other Comprehensive Income - Ending Balance	2,477	409
Accumulated Net Unrealized Investment Gain (Loss)
Pre-tax activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains on available-for-sale securities arising during period	1,155	561
Reclassification adjustment for net losses (gains) included in net income		1
Net unrealized gains on available-for-sale securities	1,155	562
Tax effect for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains on available-for-sale securities arising during period	(273)	(131)
Net unrealized gains on available-for-sale securities	(273)	(131)
Net activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during the period	882	430
Reclassification adjustment for net (gains) losses included in net income		1
Net unrealized gains on available-for-sale securities	882	431
Total Other Comprehensive Activity
Other comprehensive income, net of tax	882	431
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	812	(227)
Other comprehensive income (loss), net of tax	882	431
Total Accumulated Other Comprehensive Income - Ending Balance	1,694	204
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Pre-tax activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains on cash flow hedge derivatives arising during period	541	110
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(32)	2
Net unrealized gains on cash flow hedge derivatives	509	112
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains on cash flow hedge derivatives arising during period	(114)	(23)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	7	0
Net unrealized gains on cash flow hedge derivatives	(107)	(23)
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during the period	427	87
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(25)	2
Net unrealized gains on cash flow hedge derivatives	402	89
Total Other Comprehensive Activity
Other comprehensive income, net of tax	402	89
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	422	160
Other comprehensive income (loss), net of tax	402	89
Total Accumulated Other Comprehensive Income - Ending Balance	824	249
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	1	1
Defined benefit plans, net	1	1
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, net of tax	1	1
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(42)	(45)
Other comprehensive income (loss), net of tax	1	1
Total Accumulated Other Comprehensive Income - Ending Balance	$ (41)	$ (44)