AOCI (Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 21	$ (130)	$ (227)
Reclassification adjustment for net (gains) losses included in net income	4	(7)	(10)
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	(7)	19	48
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(12)	(31)	(49)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	(1)	1	5
Reclassification of amounts to net periodic benefit costs	(7)	(12)	(11)
Total Other Comprehensive Activity
Pre-tax activity total	(1)	(209)	(358)
Total, Tax	15	71	126
Other comprehensive income (loss)	14	(138)	(232)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	59	197	429
Other comprehensive income (loss), Net of Tax	14	(138)	(232)
Total Accumulated Other Comprehensive Income - Ending Balance	73	59	197
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	14	(196)	(349)
Net unrealized gains on available-for-sale securities	17	(207)	(365)
Tax effect for accumulated net unrealized gain (loss) on available-for-sale securities
Unrealized holding gains on available-for-sale securities arising during period	7	66	122
Reclassification adjustment for net losses (gains) included in net income	1	4	6
Net unrealized gains on available-for-sale securities	8	70	128
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	21	(130)	(227)
Reclassification adjustment for net (gains) losses included in net income	4	(7)	(10)
Net unrealized gains on available-for-sale securities	25	(137)	(237)
Total Other Comprehensive Activity
Other comprehensive income (loss)	25	(137)	(237)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	101	238	475
Other comprehensive income (loss), Net of Tax	25	(137)	(237)
Total Accumulated Other Comprehensive Income - Ending Balance	126	101	238
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 on cash flow hedge derivatives arising during period	(11)	30	74
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(19)	(48)	(75)
Net unrealized gains on cash flow hedge derivatives	(30)	(18)	(1)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains on cash flow hedge derivatives arising during period	4	(11)	(26)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	7	17	26
Net unrealized gains on cash flow hedge derivatives	11	6	0
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	(7)	19	48
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	(12)	(31)	(49)
Net unrealized gains on cash flow hedge derivatives	(19)	(12)	(1)
Total Other Comprehensive Activity
Other comprehensive income (loss)	(19)	(12)	(1)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	10	22	23
Other comprehensive income (loss), Net of Tax	(19)	(12)	(1)
Total Accumulated Other Comprehensive Income - Ending Balance	(9)	10	22
Accumulated Defined Benefit Plans Adjustment
Pre-tax activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	1	(2)	(9)
Reclassification of amounts to net periodic benefit costs	11	18	17
Defined benefit plans, net	12	16	8
Tax effect for defined benefit plans, net
Net actuarial loss	0	1	4
Reclassification of amounts to net periodic benefit costs	(4)	(6)	(6)
Defined benefit plans, net	(4)	(5)	(2)
Net activity for defined benefit plans, net
Net actuarial gain (loss) arising during the period	1	(1)	(5)
Reclassification of amounts to net periodic benefit costs	7	12	11
Defined benefit plans, net	8	11	6
Total Other Comprehensive Activity
Other comprehensive income (loss)	(8)	(11)	(6)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	(52)	(63)	(69)
Other comprehensive income (loss), Net of Tax	(8)	(11)	(6)
Total Accumulated Other Comprehensive Income - Ending Balance	$ (44)	$ (52)	$ (63)