AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 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 period	$ (167)	$ 93		$ (627)	$ 108
Reclassification adjustment for net (gains) losses included in net income	0	0		7	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	3	5		(4)	2
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0	(4)		(1)	(9)
Net activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	(1)	(1)		(2)	(2)
Total Other Comprehensive Activity
Pre-tax activity total	(212)	152		(806)	164
Total, Tax	49	(57)		183	(60)
Other comprehensive income (loss)	(163)	95	$ (623)	(623)	104
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(389)	68		73	59
Other comprehensive income (loss), Net of Tax	(163)	95	(623)	(623)	104
Total Accumulated Other Comprehensive Income - Ending Balance	(552)	163	(552)	(552)	163
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	(217)	148		(811)	170
Reclassification adjustment for net losses (gains) included in net income	0	0		9	2
Net unrealized gains on available-for-sale securities	(217)	148		(802)	172
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	50	(55)		184	(62)
Reclassification adjustment for net losses (gains) included in net income	0	0		(2)	(1)
Net unrealized gains on available-for-sale securities	50	(55)		182	(63)
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	(167)	93		(627)	108
Reclassification adjustment for net (gains) losses included in net income	0	0		7	1
Net unrealized gains on available-for-sale securities	(167)	93		(620)	109
Total Other Comprehensive Activity
Other comprehensive income (loss)	(167)	93		(620)	109
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(318)	117		135	101
Other comprehensive income (loss), Net of Tax	(167)	93		(620)	109
Total Accumulated Other Comprehensive Income - Ending Balance	(485)	210	(485)	(485)	210
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	4	8		(5)	3
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0	(6)		(1)	(14)
Net unrealized gains on cash flow hedge derivatives	4	2		(6)	(11)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	(1)	(3)		1	(1)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0	2		0	5
Net unrealized gains on cash flow hedge derivatives	(1)	(1)		1	4
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	3	5		(4)	2
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0	(4)		(1)	(9)
Net unrealized gains on cash flow hedge derivatives	3	1		(5)	(7)
Total Other Comprehensive Activity
Other comprehensive income (loss)	3	1		(5)	(7)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(19)	2		(11)	10
Other comprehensive income (loss), Net of Tax	3	1		(5)	(7)
Total Accumulated Other Comprehensive Income - Ending Balance	(16)	3	(16)	(16)	3
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	1	2		2	3
Defined benefit plans, net	1	2		2	3
Tax effect for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	0	(1)		0	(1)
Defined benefit plans, net	0	(1)		0	(1)
Net activity for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	1	1		2	2
Defined benefit plans, net	1	1		2	2
Total Other Comprehensive Activity
Other comprehensive income (loss)	(1)	(1)		(2)	(2)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(52)	(51)		(53)	(52)
Other comprehensive income (loss), Net of Tax	(1)	(1)		(2)	(2)
Total Accumulated Other Comprehensive Income - Ending Balance	$ (51)	$ (50)	$ (51)	$ (51)	$ (50)