Securities - Fair Value of Debt Securities Available for Sale by Contractual Maturity (Details) $ in Millions	Mar. 31, 2019 USD ($)
Securities available for sale
One Year or Less	$ 14
Over One Year Through Five Years	2,243
Over Five Years Through Ten Years	4,451
Over Ten Years	11,099
Total Fair Value	17,807
U.S. Treasury
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	1,301
Over Five Years Through Ten Years	2,185
Over Ten Years	0
Total Fair Value	3,486
US Government Agencies Debt Securities [Member]
Securities available for sale
One Year or Less	1
Over One Year Through Five Years	0
Over Five Years Through Ten Years	0
Over Ten Years	0
Total Fair Value	1
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	341
Over Five Years Through Ten Years	1,272
Over Ten Years	7,474
Total Fair Value	9,087
Privately issued
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	0
Over Five Years Through Ten Years	0
Over Ten Years	932
Total Fair Value	932
Privately issued - commercial mortgage-backed securities
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	33
Over Five Years Through Ten Years	74
Over Ten Years	1,346
Total Fair Value	1,453
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	339
Over Ten Years	1,199
Total Fair Value	1,538
Other
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	4
Over Five Years Through Ten Years	0
Over Ten Years	0
Total Fair Value	4
Asset Liability Management securities
Securities available for sale
One Year or Less	1
Over One Year Through Five Years	1,679
Over Five Years Through Ten Years	3,870
Over Ten Years	10,951
Total Fair Value	16,501
Direct bank purchase bonds
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	414
Over Five Years Through Ten Years	581
Over Ten Years	129
Total Fair Value	1,124
Other
Securities available for sale
One Year or Less	13
Over One Year Through Five Years	150
Over Five Years Through Ten Years	0
Over Ten Years	19
Total Fair Value	$ 182