Business Segment Reporting - Additional Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Branch industry_sector Segment	Dec. 31, 2018 USD ($) Segment
Segment Reporting Information [Line Items]
Number of operating segments | Segment	2	2
Goodwill	$ 2,516	$ 2,516
Consumer Bank [Member]
Segment Reporting Information [Line Items]
Goodwill	$ 1,600
Number Of State Branch Network | Branch	15
Commercial Bank [Member]
Segment Reporting Information [Line Items]
Goodwill	$ 912
Number Of Industrial Sectors | industry_sector	7