Allowance for Credit Losses - Analysis of Past Due and Nonaccrual Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total	$ 4,379,062	$ 4,307,366
Commercial
Financing Receivable, Past Due [Line Items]
Total	290,334
Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total		509,508
Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Total	27,110	28,586
Other | Consumer
Financing Receivable, Past Due [Line Items]
Total	82,427	82,656
Commercial | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total	2,889,183	2,818,782
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total		283,707
Commercial | Construction
Financing Receivable, Past Due [Line Items]
Total	199,381	203,681
Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Total	341,461	334,300
Residential | Construction
Financing Receivable, Past Due [Line Items]
Total	42,333	46,146
Residential | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Total	506,833
Financial Asset Originated
Financing Receivable, Past Due [Line Items]
Past due	29,339	10,720
Current	4,349,723	4,296,646
Total	4,379,062	4,307,366
Financial Asset Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	24,865	6,625
Current	3,371,151	3,321,665
Total	3,396,016	3,328,290
Financial Asset Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	2,458	3,121
Current	448,540	442,421
Total	450,998	445,542
Financial Asset Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	2,016	974
Current	288,318	282,733
Total	290,334	283,707
Financial Asset Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Current	241,714	249,827
Total	241,714	249,827
Financial Asset Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	2,462	3,477
Current	504,371	506,031
Total	506,833	509,508
Financial Asset Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	457	523
Current	26,653	28,063
Total	27,110	28,586
Financial Asset Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	226	196
Current	82,201	82,460
Total	82,427	82,656
Financial Asset Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	22,403	3,148
Current	2,866,780	2,815,634
Total	2,889,183	2,818,782
Financial Asset Originated | Commercial | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Current	199,381	203,681
Total	199,381	203,681
Financial Asset Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	1,775	2,402
Current	339,686	331,898
Total	341,461	334,300
Financial Asset Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Current	42,333	46,146
Total	42,333	46,146
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated
Financing Receivable, Past Due [Line Items]
Past due	21,030	3,812
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	19,144	1,730
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	872	1,430
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	1,014	652
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	699	1,149
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	200	175
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	100	172
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	18,445	581
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Commercial | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	572	1,083
Financial Asset, 30 to 59 Days Past Due | Financial Asset Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated
Financing Receivable, Past Due [Line Items]
Past due	2,376	1,385
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,283	507
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	161	580
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	932	298
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	0	371
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	64	216
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	12	1
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,283	136
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Commercial | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	85	363
Financial Asset, 60 to 89 Days Past Due | Financial Asset Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated
Financing Receivable, Past Due [Line Items]
Past due	5,933	5,523
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	4,438	4,388
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Consumer
Financing Receivable, Past Due [Line Items]
Past due	1,425	1,111
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Commercial
Financing Receivable, Past Due [Line Items]
Past due	70	24
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	1,763	1,957
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Home equity loans | Consumer
Financing Receivable, Past Due [Line Items]
Past due	193	132
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Other | Consumer
Financing Receivable, Past Due [Line Items]
Past due	114	23
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Past Due [Line Items]
Past due	2,675	2,431
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Commercial | Construction
Financing Receivable, Past Due [Line Items]
Past due	0	0
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Home equity lines of credit | Consumer
Financing Receivable, Past Due [Line Items]
Past due	1,118	956
Financial Asset, Equal to or Greater than 90 Days Past Due | Financial Asset Originated | Residential | Construction
Financing Receivable, Past Due [Line Items]
Past due	$ 0	$ 0