Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Interest-bearing deposits with banks	$ 119	$ 82
U.S. Treasury and federal agencies	369	255
State and political subdivisions	12	52
Other investments	122	159
Securities purchased under resale agreements	98	77
Loans and leases	198	155
Other interest-earning assets	109	77
Total interest income	1,027	857
Interest-bearing deposits	171	63
Securities sold under repurchase agreements	12	0
Other short-term borrowings	4	3
Long-term debt	106	97
Other interest-bearing liabilities	61	51
Total interest expense	354	214
Net interest income	673	643	$ 673	$ 643
Processing fees and other	$ 191	77
Effect of Change
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net interest income		$ 15