Allowance for Credit Losses - Interest Income Recognized on Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	$ 754,197	$ 748,598	$ 769,259
Interest income recognized, Total	41,461	31,983	31,397
Interest income recognized, Cash basis	29,565	21,316	21,885
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	194,451	207,616	175,877
Interest income recognized, Total	10,880	4,497	4,878
Interest income recognized, Cash basis	10,880	4,497	4,878
Commercial, Financial, Leasing, etc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	263,018	240,157	277,647
Interest income recognized, Total	7,873	3,894	8,342
Interest income recognized, Cash basis	7,873	3,894	8,342
Residential Builder and Developer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	8,699	16,209	29,237
Interest income recognized, Total	1,779	6,419	2,300
Interest income recognized, Cash basis	1,779	6,419	2,300
Other Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	11,467	15,142	19,697
Interest income recognized, Total	3,474	1,001	644
Interest income recognized, Cash basis	3,474	1,001	644
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	129,593	110,646	98,394
Interest income recognized, Total	8,386	7,177	6,227
Interest income recognized, Cash basis	3,456	3,406	3,154
Residential Limited Documentation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	82,854	93,097	103,060
Interest income recognized, Total	6,118	5,981	5,999
Interest income recognized, Cash basis	1,723	1,607	1,975
Home Equity Lines and Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	48,591	47,323	36,493
Interest income recognized, Total	1,698	1,681	1,325
Interest income recognized, Cash basis	289	400	410
Recreational Finance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	1,849	1,041	2,549
Interest income recognized, Total	333	212	147
Interest income recognized, Cash basis	9	9	49
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	9,262	15,045	19,636
Interest income recognized, Total	690	1,025	1,242
Interest income recognized, Cash basis	69	81	99
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment	4,413	2,322	6,669
Interest income recognized, Total	230	96	293
Interest income recognized, Cash basis	$ 13	$ 2	$ 34