AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during period	$ 22	$ (113)	$ 130	$ 538	[1]
Reclassification adjustment for net (gains) losses included in net income	1	(1)	2	(11)	[1]
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	0	(15)	2	49	[1]
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(2)	(7)	(11)	(23)	[1]
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	2	3	3	3
Reclassification of amounts to net periodic benefit costs	(3)	(5)	(6)	(8)	[1]
Total Other Comprehensive Activity
Pre-tax activity total	34	(182)	198	884
Total, Tax	(12)	48	(72)	(326)
Other comprehensive income (loss)	22	(134)	126	558	[1]
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		889	59	197
Other comprehensive income (loss), Net of Tax	22	(134)	126	558	[1]
Total Accumulated Other Comprehensive Income - Ending Balance	185	755	185	755
Accumulated Net Unrealized Investment Gain (Loss)
Pre-tax activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during period	34	(149)	204	855
Reclassification adjustment for net losses (gains) included in net income	1	(2)	3	(18)
Net unrealized gains on available-for-sale securities	35	(151)	207	837
Tax effect for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during period	(12)	36	(74)	(317)
Reclassification adjustment for net losses (gains) included in net income	0	1	(1)	7
Net unrealized gains on available-for-sale securities	(12)	37	(75)	(310)
Net activity for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains (losses) on available-for-sale securities arising during period	22	(113)	130	538
Reclassification adjustment for net (gains) losses included in net income	1	(1)	2	(11)
Net unrealized gains on available-for-sale securities	23	(114)	132	527
Total Other Comprehensive Activity
Other comprehensive income (loss)	23	(114)	132	527
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		879	101	238
Other comprehensive income (loss), Net of Tax	23	(114)	132	527
Total Accumulated Other Comprehensive Income - Ending Balance	233	765	233	765
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	0	(23)	3	77
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(3)	(11)	(17)	(37)
Net unrealized gains on cash flow hedge derivatives	(3)	(34)	(14)	40
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	0	8	(1)	(28)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	1	4	6	14
Net unrealized gains on cash flow hedge derivatives	1	12	5	(14)
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	0	(15)	2	49
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(2)	(7)	(11)	(23)
Net unrealized gains on cash flow hedge derivatives	(2)	(22)	(9)	26
Total Other Comprehensive Activity
Other comprehensive income (loss)	(2)	(22)	(9)	26
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		70	10	22
Other comprehensive income (loss), Net of Tax	(2)	(22)	(9)	26
Total Accumulated Other Comprehensive Income - Ending Balance	1	48	1	48
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Net actuarial loss	(3)	(5)	(4)	(5)
Reclassification of amounts to net periodic benefit costs	5	8	9	12
Defined benefit plans, net	2	3	5	7
Tax effect for defined benefit plans, net
Net actuarial loss	1	2	1	2
Reclassification of amounts to net periodic benefit costs	(2)	(3)	(3)	(4)
Defined benefit plans, net	(1)	(1)	(2)	(2)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	(2)	(3)	(3)	(3)
Reclassification of amounts to net periodic benefit costs	3	5	6	8
Defined benefit plans, net	1	2	3	5
Total Other Comprehensive Activity
Other comprehensive income (loss)	(1)	(2)	(3)	(5)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance		(60)	(52)	(63)
Other comprehensive income (loss), Net of Tax	(1)	(2)	(3)	(5)
Total Accumulated Other Comprehensive Income - Ending Balance	$ (49)	$ (58)	$ (49)	$ (58)

[1] Net tax deficiencies of $6 million were reclassified from capital surplus to applicable income tax expense for the nine months ended September 30, 2016, related to the early adoption of ASU 2016-09 during the fourth quarter of 2016, with an e ffective date of January 1, 2016.