Business Combination - Additional Information (Detail) $ / shares in Units, $ in Millions		12 Months Ended
	Mar. 22, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Aug. 26, 2019 $ / shares shares	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Total Assets		$ 169,369	$ 146,069
Percentage of common stock acquired						0.00%	3.30%	4.90%	8.60%
Full-service Banking Centers		1,149
Total noninterest expense		$ 4,660	3,958	$ 3,782
Class B Preferred stock, Series A
Business Acquisition [Line Items]
Preferred stock, issued | shares					200,000
Preferred stock, liquidation preference | $ / shares					$ 1,000
Fixed dividend rate					6.00%
Class C Preferred stock
Business Acquisition [Line Items]
Fixed dividend rate	6.00%
MB Financial, Inc.
Business Acquisition [Line Items]
Total consideration paid	$ 3,600	3,608
Total Assets			$ 20,000
Percentage of common stock acquired	100.00%
Full-service Banking Centers			86
Fair value of noncontrolling interest in acquiree	$ 197	197
Goodwill	1,800	1,777
Tax deductible goodwill	15
Total noninterest expense		222	$ 31
Contractual balances on the purchased non credit impaired loans and leases	12,700
Fair value of the purchased non credit impaired loans and leases	$ 12,500
Fair value for PCI		551
Cash consideration transferred per share | $ / shares	$ 5.54
Share consideration transferred | shares	1.45
Total value per share | $ / shares	$ 42.49
Closing price of common stock | $ / shares	$ 25.48
Contractual balances on PCI		$ 764
Locations			91
Percentage of voting equity interest	95.00%
Tax deductible goodwill, period	15 years