Securities - Fair Value of Debt Securities Available for Sale by Contractual Maturity (Details) $ in Millions	Jun. 30, 2018 USD ($)
Securities available for sale
One Year or Less	$ 82
Over One Year Through Five Years	893
Over Five Years Through Ten Years	5,658
Over Ten Years	9,384
Total Fair Value	16,017
U.S. Treasury
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	96
Over Five Years Through Ten Years	3,271
Over Ten Years	0
Total Fair Value	3,367
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	228
Over Five Years Through Ten Years	1,340
Over Ten Years	6,224
Total Fair Value	7,792
Privately issued
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	1
Over Five Years Through Ten Years	1
Over Ten Years	872
Total Fair Value	874
Privately issued - commercial mortgage-backed securities
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	0
Over Five Years Through Ten Years	42
Over Ten Years	902
Total Fair Value	944
Collateralized loan obligations
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	0
Over Five Years Through Ten Years	361
Over Ten Years	1,126
Total Fair Value	1,487
Other
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	5
Over Five Years Through Ten Years	0
Over Ten Years	0
Total Fair Value	5
Asset Liability Management securities
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	330
Over Five Years Through Ten Years	5,015
Over Ten Years	9,124
Total Fair Value	14,469
Direct bank purchase bonds
Securities available for sale
One Year or Less	74
Over One Year Through Five Years	443
Over Five Years Through Ten Years	643
Over Ten Years	239
Total Fair Value	1,399
Other
Securities available for sale
One Year or Less	8
Over One Year Through Five Years	120
Over Five Years Through Ten Years	0
Over Ten Years	21
Total Fair Value	$ 149