AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 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 period	$ (460)		$ 15
Reclassification adjustment for net (gains) losses included in net income	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	(7)		(3)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(1)		(5)
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	(594)		13
Total, Tax	134		(4)
Other comprehensive income (loss)	(460)	$ (460)	9
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	73		59
Other comprehensive income (loss), Net of Tax	(460)	(460)	9
Total Accumulated Other Comprehensive Income - Ending Balance	(389)	(389)	68
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	(594)		22
Reclassification adjustment for net losses (gains) included in net income	9		2
Net unrealized gains on available-for-sale securities	(585)		24
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	134		(7)
Reclassification adjustment for net losses (gains) included in net income	(2)		(1)
Net unrealized gains on available-for-sale securities	132		(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 period	(460)		15
Reclassification adjustment for net (gains) losses included in net income	7		1
Net unrealized gains on available-for-sale securities	(453)		16
Total Other Comprehensive Activity
Other comprehensive income (loss)	(453)		16
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	135		101
Other comprehensive income (loss), Net of Tax	(453)		16
Total Accumulated Other Comprehensive Income - Ending Balance	(318)	(318)	117
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	(9)		(5)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(1)		(8)
Net unrealized gains on cash flow hedge derivatives	(10)		(13)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	2		2
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0		3
Net unrealized gains on cash flow hedge derivatives	2		5
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	(7)		(3)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(1)		(5)
Net unrealized gains on cash flow hedge derivatives	(8)		(8)
Total Other Comprehensive Activity
Other comprehensive income (loss)	(8)		(8)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(11)		10
Other comprehensive income (loss), Net of Tax	(8)		(8)
Total Accumulated Other Comprehensive Income - Ending Balance	(19)	(19)	2
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Net actuarial loss	0
Reclassification of amounts to net periodic benefit costs	1		2
Defined benefit plans, net	1		2
Tax effect for defined benefit plans, net
Net actuarial loss	0
Reclassification of amounts to net periodic benefit costs	0		(1)
Defined benefit plans, net	0		(1)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	0
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	(53)		(52)
Other comprehensive income (loss), Net of Tax	(1)		(1)
Total Accumulated Other Comprehensive Income - Ending Balance	$ (52)	$ (52)	$ (51)