Loans and Allowance for Credit Losses - Schedule of Composition of Loans and Impact of Adoption (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 12,384,612	$ 12,117,524	$ 12,117,524
Allowance	(106,380)	(54,619)	(54,619)	$ (55,559)	$ (55,461)
Net loans	12,278,232	12,062,905
Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		4,478
Allowance		(41,347)
Net loans		(36,869)
Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		12,122,002	12,117,524
Allowance		(95,966)	(54,619)
Net loans		12,026,036
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,844,465	2,815,154	2,890,296
Allowance	(31,125)	(21,359)	(22,585)	(20,406)	(21,742)
Commercial | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2,679
Allowance		(7,150)
Commercial | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2,817,833	2,815,154
Allowance		(28,509)	(21,359)
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	5,118,439	4,889,253	5,166,792
Allowance	(54,103)	(20,535)	(21,588)	(25,169)	(23,470)
Commercial real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		1,637
Allowance		(25,548)
Commercial real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		4,890,890	4,889,253
Allowance		(46,083)	(20,535)
BBCC
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	367,139	352,681
Allowance	(5,417)	(2,279)
BBCC | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		33
Allowance		(3,702)
BBCC | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		352,714	352,681
Allowance		(5,981)	(2,279)
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,327,851	2,334,289	2,334,289
Allowance	(9,637)	(2,299)	(2,299)	(2,302)	(2,277)
Residential real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		105
Allowance		(6,986)
Residential real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2,334,394	2,334,289
Allowance		(9,285)	(2,299)
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans			1,726,147
Allowance			(8,147)	$ (7,682)	$ (7,972)
Indirect
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	953,136	935,584
Allowance	(3,666)	(5,319)
Indirect | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		10
Allowance		1,669
Indirect | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		935,594	935,584
Allowance		(3,650)	(5,319)
Direct
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	211,793	228,524
Allowance	(822)	(1,863)
Direct | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2
Allowance		1,059
Direct | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		228,526	228,524
Allowance		(804)	(1,863)
Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	561,789	562,039
Allowance	(1,610)	(965)
Home equity | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		12
Allowance		(689)
Home equity | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		562,051	562,039
Allowance		(1,654)	(965)
Direct Finance Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 44,400		47,200
Previously Reported
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		12,117,524	12,117,524
Allowance		(54,619)	(54,619)
Previously Reported | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2,890,296	2,890,296
Allowance		(22,585)	(22,585)
Previously Reported | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		5,166,792	5,166,792
Allowance		(21,588)	(21,588)
Previously Reported | Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		2,334,289	2,334,289
Allowance		(2,299)	(2,299)
Previously Reported | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		1,726,147	1,726,147
Allowance		(8,147)	(8,147)
Credit Risk Reclassification | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		(75,142)	(75,142)
Allowance		1,226	1,226
Credit Risk Reclassification | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		(277,539)	(277,539)
Allowance		1,053	1,053
Credit Risk Reclassification | BBCC
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		352,681	352,681
Allowance		(2,279)	(2,279)
Credit Risk Reclassification | Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		0	0
Allowance		0	0
Credit Risk Reclassification | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		(1,726,147)	(1,726,147)
Allowance		8,147	8,147
Credit Risk Reclassification | Indirect
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		935,584	935,584
Allowance		(5,319)	(5,319)
Credit Risk Reclassification | Direct
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		228,524	228,524
Allowance		(1,863)	(1,863)
Credit Risk Reclassification | Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans		562,039	562,039
Allowance		$ (965)	$ (965)