Recent Accounting Pronouncements - 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
Assets
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
Net deferred tax assets	17,576	29,705	29,705
Liabilities
Allowance for credit losses on unfunded loan commitments	8,950	2,656	2,656
Shareholders' equity:
Retained earnings	649,909	682,185	682,185
Impact of ASC 326 Adoption
Assets
Loans		4,478
Allowance		(41,347)
Net loans		(36,869)
Net deferred tax assets		10,268
Liabilities
Allowance for credit losses on unfunded loan commitments		4,549	4,549
Shareholders' equity:
Retained earnings		(31,150)
Adjusted Balance
Assets
Loans		12,122,002	12,117,524
Allowance		(95,966)	(54,619)
Net loans		12,026,036
Net deferred tax assets		39,973
Liabilities
Allowance for credit losses on unfunded loan commitments	7,205	7,205
Shareholders' equity:
Retained earnings		651,035
Commercial
Assets
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
Assets
Loans		2,679
Allowance		(7,150)
Commercial | Adjusted Balance
Assets
Loans		2,817,833	2,815,154
Allowance		(28,509)	(21,359)
Commercial real estate
Assets
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
Assets
Loans		1,637
Allowance		(25,548)
Commercial real estate | Adjusted Balance
Assets
Loans		4,890,890	4,889,253
Allowance		(46,083)	(20,535)
BBCC
Assets
Loans	367,139	352,681
Allowance	(5,417)	(2,279)
BBCC | Impact of ASC 326 Adoption
Assets
Loans		33
Allowance		(3,702)
BBCC | Adjusted Balance
Assets
Loans		352,714	352,681
Allowance		(5,981)	(2,279)
Residential real estate
Assets
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
Assets
Loans		105
Allowance		(6,986)
Residential real estate | Adjusted Balance
Assets
Loans		2,334,394	2,334,289
Allowance		(9,285)	(2,299)
Consumer
Assets
Loans			1,726,147
Allowance			(8,147)	$ (7,682)	$ (7,972)
Indirect
Assets
Loans	953,136	935,584
Allowance	(3,666)	(5,319)
Indirect | Impact of ASC 326 Adoption
Assets
Loans		10
Allowance		1,669
Indirect | Adjusted Balance
Assets
Loans		935,594	935,584
Allowance		(3,650)	(5,319)
Direct
Assets
Loans	211,793	228,524
Allowance	(822)	(1,863)
Direct | Impact of ASC 326 Adoption
Assets
Loans		2
Allowance		1,059
Direct | Adjusted Balance
Assets
Loans		228,526	228,524
Allowance		(804)	(1,863)
Home equity
Assets
Loans	561,789	562,039
Allowance	$ (1,610)	(965)
Home equity | Impact of ASC 326 Adoption
Assets
Loans		12
Allowance		(689)
Home equity | Adjusted Balance
Assets
Loans		562,051	562,039
Allowance		(1,654)	(965)
Previously Reported
Assets
Loans		12,117,524	12,117,524
Allowance		(54,619)	(54,619)
Previously Reported | Commercial
Assets
Loans		2,890,296	2,890,296
Allowance		(22,585)	(22,585)
Previously Reported | Commercial real estate
Assets
Loans		5,166,792	5,166,792
Allowance		(21,588)	(21,588)
Previously Reported | Residential real estate
Assets
Loans		2,334,289	2,334,289
Allowance		(2,299)	(2,299)
Previously Reported | Consumer
Assets
Loans		1,726,147	1,726,147
Allowance		(8,147)	(8,147)
Segment Portfolio Reclassifications | Commercial
Assets
Loans		(75,142)	(75,142)
Allowance		1,226	1,226
Segment Portfolio Reclassifications | Commercial real estate
Assets
Loans		(277,539)	(277,539)
Allowance		1,053	1,053
Segment Portfolio Reclassifications | BBCC
Assets
Loans		352,681	352,681
Allowance		(2,279)	(2,279)
Segment Portfolio Reclassifications | Residential real estate
Assets
Loans		0	0
Allowance		0	0
Segment Portfolio Reclassifications | Consumer
Assets
Loans		(1,726,147)	(1,726,147)
Allowance		8,147	8,147
Segment Portfolio Reclassifications | Indirect
Assets
Loans		935,584	935,584
Allowance		(5,319)	(5,319)
Segment Portfolio Reclassifications | Direct
Assets
Loans		228,524	228,524
Allowance		(1,863)	(1,863)
Segment Portfolio Reclassifications | Home equity
Assets
Loans		562,039	562,039
Allowance		$ (965)	$ (965)