AOCI - Activity of the Components of Other Comprehensive Income and AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2018
Net activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period	$ 577	$ (167)		$ 1,007		$ (627)
Reclassification adjustment for net (gains) losses included in net income	0	0		1		7
Net activity for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	190	3		277		(4)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	1	0		3		(1)
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	1,000	(212)		1,676		(806)
Total, Tax	(231)	49		(386)		183
Other comprehensive income (loss)	769	(163)	$ 1,290	1,290	$ (623)	(623)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	409	(389)		(112)		71
Other comprehensive income (loss), Net of Tax	769	(163)	1,290	1,290	(623)	(623)
Total Accumulated Other Comprehensive Income - Ending Balance	1,178	(552)	1,178	1,178	(552)	(552)
Accumulated Net Unrealized Investment Gain (Loss)
Pre-tax activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period	758	(217)		1,319		(811)
Reclassification adjustment for net losses (gains) included in net income	0	0		(1)		9
Net unrealized gains on available-for-sale debt securities	758	(217)		1,320		(802)
Tax effect for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period	(181)	50		(312)		184
Reclassification adjustment for net losses (gains) included in net income	0	0		0		(2)
Net unrealized gains on available-for-sale debt securities	(181)	50		(312)		182
Net activity for accumulated net unrealized gain (loss) on available-for-sale debt securities
Unrealized holding gains (losses) on available-for-sale debt securities arising during period	577	(167)		1,007		(627)
Reclassification adjustment for net (gains) losses included in net income	0	0		1		7
Net unrealized gains on available-for-sale debt securities	577	(167)		1,008		(620)
Total Other Comprehensive Activity
Other comprehensive income (loss)	577	(167)		1,008		(620)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	204	(318)		(227)		135
Other comprehensive income (loss), Net of Tax	577	(167)		1,008		(620)
Total Accumulated Other Comprehensive Income - Ending Balance	781	(485)	781	781	(485)	(485)
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	240	4		351		(5)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	1	0		3		(1)
Net unrealized gains on cash flow hedge derivatives	241	4		354		(6)
Tax effect for net unrealized gain (loss) on cash flow hedge derivatives
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	(50)	(1)		(74)		1
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	0	0		0		0
Net unrealized gains on cash flow hedge derivatives	(50)	(1)		(74)		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	190	3		277		(4)
Reclassification adjustment for net gains on cash flow hedge derivatives included in net income	1	0		3		(1)
Net unrealized gains on cash flow hedge derivatives	191	3		280		(5)
Total Other Comprehensive Activity
Other comprehensive income (loss)	191	3		280		(5)
Total Accumulated Other Comprehensive Income
Total Accumulated Other Comprehensive Income - Beginning Balance	249	(19)		160		(11)
Other comprehensive income (loss), Net of Tax	191	3		280		(5)
Total Accumulated Other Comprehensive Income - Ending Balance	440	(16)	440	440	(16)	(16)
Amortization Of Defined Benefit Pension Items
Pre-tax 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
Tax effect for defined benefit plans, net
Reclassification of amounts to net periodic benefit costs	0	0		0		0
Defined benefit plans, net	0	0		0		0
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	(44)	(52)		(45)		(53)
Other comprehensive income (loss), Net of Tax	1	(1)		2		2
Total Accumulated Other Comprehensive Income - Ending Balance	$ (43)	$ (51)	$ (43)	$ (43)	$ (51)	$ (51)