Business Combination - Additional Information (Detail) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 22, 2019 USD ($) $ / shares shares	Mar. 31, 2020 USD ($) centers	Mar. 31, 2019 USD ($)		Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Aug. 26, 2019 $ / shares shares	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Total Assets		$ 185,391.0				$ 169,369.0
Full-service Banking | centers		1,123
Total noninterest expense		$ 1,200.0	$ 1,097.0	[1]
Allowance for loan and lease losses		2,348.0	1,115.0			1,202.0		$ 1,103.0
Financing Receivable, Net Amount		115,689.0				$ 108,356.0
Purchased Credit Deteriorated [Member]
Business Acquisition [Line Items]
Financing Receivable, Net Amount		508.0
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	$ 3,608.0
Total Assets								$ 20,000.0
Percentage of common stock acquired	100.00%
Full-service Banking								86
Fair value of noncontrolling interest in acquiree	$ 197.0
Goodwill	1,786.0
Tax deductible goodwill	$ 15.0
Total noninterest expense		7.0	76.0
Contractual balances on the purchased non credit impaired loans and leases		12.7
Fair value of the purchased non credit impaired loans and leases		$ 12.5
Acquisition date through period end, net income			13.0
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
Locations								91
Percentage of voting equity interest	95.00%
Tax deductible goodwill, period	15 years
MB Financial, Inc. | Net Interest Income [Member]
Business Acquisition [Line Items]
Acquisition date through period end, revenue			16.0
MB Financial, Inc. | Non Interest Income [Member]
Business Acquisition [Line Items]
Acquisition date through period end, revenue			$ 12.0
ASU 2016-13
Business Acquisition [Line Items]
Allowance for loan and lease losses					$ 33.0
Unpaid principal balance as noncredit discount					$ 87.0

[1] During the first quarter of 2020, certain noninterest income and noninterest expense line items were reclassified to better align disclosures to business activities. These reclassifications were retrospectively applied to all prior periods presented. Total noninterest income and noninterest expense did not change as a result of these reclassifications.