Allowance for Credit Losses - Interest Income Recognized on Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	$ 748,598	$ 769,259	$ 768,501
Interest Income Recognized, Total	31,983	31,397	27,643
Interest Income Recognized, Cash Basis	21,316	21,885	19,046
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	207,616	175,877	166,628
Interest Income Recognized, Total	4,497	4,878	6,243
Interest Income Recognized, Cash Basis	4,497	4,878	6,243
Commercial, Financial, Leasing, etc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	240,157	277,647	236,201
Interest Income Recognized, Total	3,894	8,342	2,933
Interest Income Recognized, Cash Basis	3,894	8,342	2,933
Residential Builder and Developer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	16,209	29,237	59,457
Interest Income Recognized, Total	6,419	2,300	335
Interest Income Recognized, Cash Basis	6,419	2,300	335
Other Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	15,142	19,697	20,276
Interest Income Recognized, Total	1,001	644	2,311
Interest Income Recognized, Cash Basis	1,001	644	2,311
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	110,646	98,394	101,483
Interest Income Recognized, Total	7,177	6,227	6,188
Interest Income Recognized, Cash Basis	3,406	3,154	4,037
Residential Limited Documentation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	93,097	103,060	118,449
Interest Income Recognized, Total	5,981	5,999	6,380
Interest Income Recognized, Cash Basis	1,607	1,975	2,638
Home Equity Lines and Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	47,323	36,493	21,523
Interest Income Recognized, Total	1,681	1,325	905
Interest Income Recognized, Cash Basis	400	410	261
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	15,045	19,636	25,675
Interest Income Recognized, Total	1,025	1,242	1,619
Interest Income Recognized, Cash Basis	81	99	175
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	3,363	9,218	18,809
Interest Income Recognized, Total	308	440	729
Interest Income Recognized, Cash Basis	$ 11	$ 83	$ 113