Securities - Fair Value of Debt Securities Available for Sale by Contractual Maturity (Details) $ in Millions	Sep. 30, 2017 USD ($)
Securities available for sale
One Year or Less	$ 19
Over One Year Through Five Years	610
Over Five Years Through Ten Years	4,959
Over Ten Years	12,516
Total Fair Value	18,104
U.S. Treasury
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	0
Over Five Years Through Ten Years	2,982
Over Ten Years	0
Total Fair Value	2,982
U.S. agency securities
Securities available for sale
One Year or Less	9
Over One Year Through Five Years	3
Over Five Years Through Ten Years	0
Over Ten Years	0
Total Fair Value	12
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities available for sale
One Year or Less	2
Over One Year Through Five Years	4
Over Five Years Through Ten Years	407
Over Ten Years	9,633
Total Fair Value	10,046
Privately issued
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	1
Over Five Years Through Ten Years	0
Over Ten Years	550
Total Fair Value	551
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	0
Over Ten Years	742
Total Fair Value	742
Collateralized loan obligations
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	12
Over Five Years Through Ten Years	940
Over Ten Years	1,189
Total Fair Value	2,141
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	11
Over One Year Through Five Years	25
Over Five Years Through Ten Years	4,329
Over Ten Years	12,114
Total Fair Value	16,479
Direct bank purchase bonds
Securities available for sale
One Year or Less	8
Over One Year Through Five Years	529
Over Five Years Through Ten Years	630
Over Ten Years	378
Total Fair Value	1,545
Other
Securities available for sale
One Year or Less	0
Over One Year Through Five Years	56
Over Five Years Through Ten Years	0
Over Ten Years	24
Total Fair Value	$ 80