AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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 year	$ 1,046	$ (371)	$ 21
Reclassification adjustment for net (gains) losses included in net income	7	(9)	(4)
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during the year	275	169	(7)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(13)	2	(12)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the year	(5)	1	1
Reclassification of amounts to net periodic benefit costs	8	7	7
Total Other Comprehensive Activity
Pre-tax activity total	1,696	(246)	(1)
Total, Tax	(392)	63	15
Other comprehensive income (loss), net of tax	1,304	(183)	14
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(112)	73
Other comprehensive income (loss), net of tax	1,304	(183)	14
Total Accumulated Other Comprehensive Income - Ending Balance	1,192	(112)	73
Derivative Hedge AOCI 2019
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(112)
Total Accumulated Other Comprehensive Income - Ending Balance		(112)
Derivative Hedge AOCI 2018
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		71
Total Accumulated Other Comprehensive Income - Ending Balance			71
Derivative Hedge AOCI 2017
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance			59
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	1,369	(483)	14
Reclassification adjustment for net losses (gains) included in net income	(9)	11	3
Net unrealized gains on available-for-sale securities	1,360	(472)	17
Tax effect for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains on available-for-sale securities arising during period	(323)	112	7
Reclassification adjustment for net losses (gains) included in net income	2	(2)	1
Net unrealized gains on available-for-sale securities	(321)	110	8
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 year	1,046	(371)	21
Reclassification adjustment for net (gains) losses included in net income	(7)	9	4
Net unrealized gains on available-for-sale securities	1,039	(362)	25
Total Other Comprehensive Activity
Other comprehensive income (loss), net of tax	1,039	(362)	25
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(227)	126
Other comprehensive income (loss), net of tax	1,039	(362)	25
Total Accumulated Other Comprehensive Income - Ending Balance	812	(227)	126
Accumulated Net Unrealized Investment Gain (Loss) | Derivative Hedge AOCI 2019
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(227)
Total Accumulated Other Comprehensive Income - Ending Balance		(227)
Accumulated Net Unrealized Investment Gain (Loss) | Derivative Hedge AOCI 2018
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		135
Total Accumulated Other Comprehensive Income - Ending Balance			135
Accumulated Net Unrealized Investment Gain (Loss) | Derivative Hedge AOCI 2017
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance			101
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	348	214	(11)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(16)	2	(19)
Net unrealized gains on cash flow hedge derivatives	332	216	(30)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains on cash flow hedge derivatives arising during period	(73)	(45)	4
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	3	0	7
Net unrealized gains on cash flow hedge derivatives	(70)	(45)	11
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during the year	275	169	(7)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(13)	2	(12)
Net unrealized gains on cash flow hedge derivatives	262	171	(19)
Total Other Comprehensive Activity
Other comprehensive income (loss), net of tax	262	171	(19)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	160	(9)
Other comprehensive income (loss), net of tax	262	171	(19)
Total Accumulated Other Comprehensive Income - Ending Balance	422	160	(9)
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Derivative Hedge AOCI 2019
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	160
Total Accumulated Other Comprehensive Income - Ending Balance		160
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Derivative Hedge AOCI 2018
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		(11)
Total Accumulated Other Comprehensive Income - Ending Balance			(11)
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Derivative Hedge AOCI 2017
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance			10
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	(5)	1	1
Reclassification of amounts to net periodic benefit costs	9	9	11
Defined benefit plans, net	4	10	12
Tax effect for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	(1)	(2)	(4)
Defined benefit plans, net	(1)	(2)	(4)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the year	(5)	1	1
Reclassification of amounts to net periodic benefit costs	8	7	7
Defined benefit plans, net	3	8	8
Total Other Comprehensive Activity
Other comprehensive income (loss), net of tax	3	8	8
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(45)	(44)
Other comprehensive income (loss), net of tax	3	8	8
Total Accumulated Other Comprehensive Income - Ending Balance	(42)	(45)	(44)
Accumulated Defined Benefit Plans Adjustment | Derivative Hedge AOCI 2019
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	$ (45)
Total Accumulated Other Comprehensive Income - Ending Balance		(45)
Accumulated Defined Benefit Plans Adjustment | Derivative Hedge AOCI 2018
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		$ (53)
Total Accumulated Other Comprehensive Income - Ending Balance			(53)
Accumulated Defined Benefit Plans Adjustment | Derivative Hedge AOCI 2017
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance			$ (52)