Line of Business Information - Summary of Line of Business (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Servicing fees	$ 1,251	$ 1,421
Management fees	420	472
Foreign exchange trading services	280	304
Securities finance	118	141
Processing fees and other	191	77
Total fee revenue	2,260	2,415
Net interest income	673	643	$ 673	$ 643
Gains (losses) related to investment securities, net	(1)	(2)
Total revenue	2,932	3,056
Total expenses	2,293	2,268
Income before income taxes	$ 635	$ 788
Pre-tax margin	22.00%	26.00%
Operating Segments | Investment Servicing
Segment Reporting Information [Line Items]
Servicing fees	$ 1,251	$ 1,421
Management fees	0	0
Foreign exchange trading services	246	274
Securities finance	117	141
Processing fees and other	180	77
Total fee revenue	1,794	1,913
Net interest income	679	648
Gains (losses) related to investment securities, net	(1)	(2)
Total revenue	2,472	2,559
Total expenses	1,864	1,870
Income before income taxes	$ 604	$ 689
Pre-tax margin	24.00%	27.00%
Operating Segments | Investment Management
Segment Reporting Information [Line Items]
Servicing fees	$ 0	$ 0
Management fees	420	472
Foreign exchange trading services	34	30
Securities finance	1	0
Processing fees and other	11	0
Total fee revenue	466	502
Net interest income	(6)	(5)
Gains (losses) related to investment securities, net	0	0
Total revenue	460	497
Total expenses	406	398
Income before income taxes	$ 54	$ 99
Pre-tax margin	12.00%	20.00%
Other
Segment Reporting Information [Line Items]
Servicing fees	$ 0	$ 0
Management fees	0	0
Foreign exchange trading services	0	0
Securities finance	0	0
Processing fees and other	0	0
Total fee revenue	0	0
Net interest income	0	0
Gains (losses) related to investment securities, net	0	0
Total revenue	0	0
Total expenses	23	0
Income before income taxes	$ (23)	$ 0