Loans and Allowance for Credit Losses - Schedule of Composition of Loans and Impact of Adoption (Detail) - USD ($) $ in Thousands	Jun. 30, 2020		Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 13,615,701			$ 12,117,524	$ 12,117,524
Allowance	(128,394)	[1]	$ (106,380)	(54,619)	(54,619)	[1]	$ (56,292)	$ (55,559)	$ (55,461)
Net loans	13,487,307			12,062,905	12,062,905
Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	13,615,701			12,122,002
Allowance	(128,394)		(106,380)	(95,966)
Net loans	13,487,307			12,026,036
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	4,307,505			2,815,154	2,890,296
Allowance	(29,618)		(31,125)	(21,359)	(22,585)		(22,548)	(20,406)	(21,742)
Commercial | PPP Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,463,000
Commercial | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	4,113,644			2,817,833
Allowance			(31,125)	(28,509)
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	5,403,316			4,889,253	5,166,792
Allowance	(71,069)		(54,103)	(20,535)	(21,588)		(23,311)	(25,169)	(23,470)
Commercial real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	5,236,947			4,890,890
Allowance			(54,103)	(46,083)
BBCC
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans				352,681
Allowance	(6,332)		(5,417)	(2,279)
BBCC | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	360,230			352,714
Allowance			(5,417)	(5,981)
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,229,298			2,334,289	2,334,289
Allowance	(14,244)		(9,637)	(2,299)	(2,299)		(2,269)	(2,302)	(2,277)
Residential real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,229,298			2,334,394
Allowance			(9,637)	(9,285)
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,675,582				1,726,147
Allowance					(8,147)		$ (8,164)	$ (7,682)	$ (7,972)
Indirect
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	933,723			935,584
Allowance	(4,453)		(3,666)	(5,319)
Indirect | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	933,723			935,594
Allowance			(3,666)	(3,650)
Direct
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	194,573			228,524
Allowance	(835)		(822)	(1,863)
Direct | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	194,573			228,526
Allowance			(822)	(804)
Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	547,286			562,039
Allowance	(1,843)		(1,610)	(965)
Home equity | Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	547,286			562,051
Allowance			$ (1,610)	(1,654)
Direct Finance Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	42,100				$ 47,200
Previously Reported
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	13,615,701			12,117,524
Allowance				(54,619)
Previously Reported | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	4,307,505			2,890,296
Allowance				(22,585)
Previously Reported | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	5,403,316			5,166,792
Allowance				(21,588)
Previously Reported | Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,229,298			2,334,289
Allowance				(2,299)
Previously Reported | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,675,582			1,726,147
Allowance				(8,147)
Segment Portfolio Reclassifications | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	(193,861)			(75,142)
Allowance				1,226
Segment Portfolio Reclassifications | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	(166,369)			(277,539)
Allowance				1,053
Segment Portfolio Reclassifications | BBCC
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	360,230			352,681
Allowance				(2,279)
Segment Portfolio Reclassifications | Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0			0
Allowance				0
Segment Portfolio Reclassifications | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	(1,675,582)			(1,726,147)
Allowance				8,147
Segment Portfolio Reclassifications | Indirect
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	933,723			935,584
Allowance				(5,319)
Segment Portfolio Reclassifications | Direct
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	194,573			228,524
Allowance				(1,863)
Segment Portfolio Reclassifications | Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 547,286			562,039
Allowance				$ (965)

[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.