Loans and Allowance for Credit Losses - Schedule of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 106,380		$ 55,559	$ 54,619	[1]	$ 55,461
Charge-offs	(2,232)		(2,876)	(10,677)		(5,769)
Recoveries	1,701		2,606	3,610		4,554
Provision for Credit Losses	22,545		1,003	39,495		2,046
Balance at End of Period	128,394	[1]	56,292	128,394	[1]	56,292
Allowance for credit losses on unfunded loan commitments	11,026			11,026			$ 8,950	$ 2,656	$ 2,656
Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Allowance for credit losses on unfunded loan commitments								4,549	4,549
Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	106,380
Balance at End of Period	128,394			128,394
Allowance for credit losses on unfunded loan commitments								$ 7,205	$ 7,205
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	31,125		20,406	22,585		21,742
Charge-offs	(136)		(604)	(5,178)		(764)
Recoveries	553		334	910		709
Provision for Credit Losses	(1,924)		2,412	5,377		861
Balance at End of Period	29,618		22,548	29,618		22,548
Commercial | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Commercial | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	31,125
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	54,103		25,169	21,588		23,470
Charge-offs	(1,160)		(389)	(2,452)		(624)
Recoveries	246		1,205	915		1,775
Provision for Credit Losses	17,880		(2,674)	26,523		(1,310)
Balance at End of Period	71,069		23,311	71,069		23,311
Commercial real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Commercial real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	54,103
BBCC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	5,417
Charge-offs	(66)			(81)
Recoveries	56			122
Provision for Credit Losses	925			310
Balance at End of Period	6,332			6,332
BBCC | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
BBCC | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	5,417
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	9,637		2,302	2,299		2,277
Charge-offs	(16)		(140)	(316)		(318)
Recoveries	42		9	211		81
Provision for Credit Losses	4,581		98	5,064		229
Balance at End of Period	14,244		2,269	14,244		2,269
Residential real estate | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Residential real estate | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	9,637
Indirect
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	3,666
Charge-offs	(367)			(1,570)
Recoveries	494			908
Provision for Credit Losses	660			1,465
Balance at End of Period	4,453			4,453
Indirect | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Indirect | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	3,666
Direct
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	822
Charge-offs	(405)			(880)
Recoveries	231			383
Provision for Credit Losses	187			528
Balance at End of Period	835			835
Direct | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Direct | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	822
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	1,610
Charge-offs	(82)			(200)
Recoveries	79			161
Provision for Credit Losses	236			228
Balance at End of Period	1,843			1,843
Home equity | Impact of ASC 326 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Home equity | Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 1,610
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period			7,682	$ 8,147		7,972
Charge-offs			(1,743)			(4,063)
Recoveries			1,058			1,989
Provision for Credit Losses			1,167			2,266
Balance at End of Period			$ 8,164			$ 8,164

[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.