Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Thousands		12 Months Ended
	Jan. 01, 2020 USD ($)	Dec. 31, 2020 USD ($)		Dec. 31, 2020 portfolio	Dec. 31, 2020 segment	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Loans held for sale, at fair value		$ 63,250				$ 46,898
Percentage of income tax examination likelihood of tax benefits		50.00%
Number of distinct loan portfolios | portfolio				7
Current expected credit losses, initial loss forecast term		1 year
Current expected credit losses, reversion period, term		2 years
Allowance for loan losses		$ 131,388	[1]			54,619	[1]	$ 55,461	$ 50,381
Shareholders' equity		(2,972,656)				(2,852,453)		(2,689,570)	(2,154,397)
Noncredit discount amount accreted into interest income	$ 11,800
Number of loan portfolios | portfolio				4
Number of loan segments				7	7
Retained Earnings
Basis Of Presentation And Significant Accounting Policies [Line Items]
Shareholders' equity		(783,892)				(682,185)		(527,684)	(413,130)
Adjustment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Allowance for loan losses	41,347					41,347
Allowance for credit losses on unfunded loan commitments	4,549
Shareholders' equity						31,150		(6,322)	4,179
Adjustment | Retained Earnings
Basis Of Presentation And Significant Accounting Policies [Line Items]
Shareholders' equity						31,150		$ (6,322)	$ 4,127
Adjustment | Retained Earnings | Accounting Standards Update 2016-13
Basis Of Presentation And Significant Accounting Policies [Line Items]
Shareholders' equity						31,200
Residential Mortgage Loans
Basis Of Presentation And Significant Accounting Policies [Line Items]
Loans held for sale, at fair value		$ 63,300				$ 46,900
Acquired Loan Portfolio | Adjustment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Allowance for loan losses	$ 27,100
Minimum
Basis Of Presentation And Significant Accounting Policies [Line Items]
Maturity of short-term securities sold under agreements to repurchase		1 day
Minimum | Core Deposits and Other Intangible Assets
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships		5 years
Minimum | Building and Building Improvements
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years		15 years
Minimum | Furniture and Equipment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years		3 years
Maximum
Basis Of Presentation And Significant Accounting Policies [Line Items]
Maturity of short-term securities sold under agreements to repurchase		4 days
Maximum | Core Deposits and Other Intangible Assets
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships		15 years
Maximum | Building and Building Improvements
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years		39 years
Maximum | Furniture and Equipment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years		7 years

[1] Beginning January 1, 2020, calculation is based on current expected loss methodology. Prior to January 1, 2020, calculation was based on incurred loss methodology.