Allowance for Credit Losses - Interest Income Recognized on Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 827,359	$ 754,197	$ 748,598
Interest income recognized, Total	33,400	41,461	31,983
Interest income recognized, Cash basis	20,619	29,565	21,316
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	208,200	194,451	207,616
Interest income recognized, Total	5,850	10,880	4,497
Interest income recognized, Cash basis	5,850	10,880	4,497
Commercial, Financial, Leasing, etc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	303,712	263,018	240,157
Interest income recognized, Total	8,960	7,873	3,894
Interest income recognized, Cash basis	8,960	7,873	3,894
Residential Builder and Developer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	10,863	8,699	16,209
Interest income recognized, Total	357	1,779	6,419
Interest income recognized, Cash basis	357	1,779	6,419
Other Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	26,440	11,467	15,142
Interest income recognized, Total	634	3,474	1,001
Interest income recognized, Cash basis	634	3,474	1,001
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	142,400	129,593	110,646
Interest income recognized, Total	9,200	8,386	7,177
Interest income recognized, Cash basis	3,567	3,456	3,406
Residential Limited Documentation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	74,399	82,854	93,097
Interest income recognized, Total	5,463	6,118	5,981
Interest income recognized, Cash basis	881	1,723	1,607
Home Equity Lines and Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	47,168	48,591	47,323
Interest income recognized, Total	1,639	1,698	1,681
Interest income recognized, Cash basis	270	289	400
Recreational Finance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	5,707	1,849	1,041
Interest income recognized, Total	558	333	212
Interest income recognized, Cash basis	17	9	9
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	3,445	9,262	15,045
Interest income recognized, Total	214	690	1,025
Interest income recognized, Cash basis	72	69	81
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	5,025	4,413	2,322
Interest income recognized, Total	525	230	96
Interest income recognized, Cash basis	$ 11	$ 13	$ 2