Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Current	$ 6,194,443	$ 5,028,841
Total Past Due	42,409	37,001
Total Loans	6,236,852	5,065,842
90 Days or More Past Due and Accruing	2,392	3,126
Loans held for sale, current	20,231	7,899
Loans held for sale	20,231	7,899
Total loans, current	6,214,674	5,036,740
Total loans	6,257,083	5,073,741
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	499,983	412,450
Total Past Due	5,386	4,439
Total Loans	505,369	416,889
90 Days or More Past Due and Accruing	413	407
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,815,523	2,245,650
Total Past Due	11,111	10,731
Total Loans	2,826,634	2,256,381
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,379,216	1,234,839
Total Past Due	16,670	12,961
Total Loans	1,395,886	1,247,800
90 Days or More Past Due and Accruing	1,482	2,159
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	406,284	401,242
Total Past Due	4,891	5,652
Total Loans	411,175	406,894
90 Days or More Past Due and Accruing	451	527
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	493,600	344,184
Total Past Due	603	564
Total Loans	494,203	344,748
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,321,923	1,901,466
Total Past Due	10,508	10,167
Total Loans	2,332,431	1,911,633
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,093,437	734,660
Total Past Due	4,351	3,218
Total Loans	1,097,788	737,878
90 Days or More Past Due and Accruing	46	33
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	8,781	11,349
Total Past Due	21,875	22,032
Total Loans	30,656	33,381
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	8,032	10,710
Total Past Due	573	838
Total Loans	8,605	11,548
Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	16,813	22,059
Total Past Due	22,448	22,870
Total Loans	39,261	44,929
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	12,616	8,963
30-59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,429	2,252
30-59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,616	909
30-59 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,224	1,389
30-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,276	4,115
30-59 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	247
30-59 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,369	909
30-59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,071	298
30-59 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,338	943
30-59 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	121	390
30-59 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,459	1,333
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	7,845	5,760
60-89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	467	314
60-89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	744	1,097
60-89 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	4,153	2,871
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	896	764
60-89 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	145
60-89 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	599	1,097
60-89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,585	714
60-89 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,364	2,147
60-89 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	69	238
60-89 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,433	2,385
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	21,948	22,278
90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,490	1,873
90 Days or More Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,751	8,725
90 Days or More Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	10,293	8,701
90 Days or More Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	719	773
90 Days or More Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	211	564
90 Days or More Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,540	8,161
90 Days or More Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,695	2,206
90 Days or More Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	19,173	18,942
90 Days or More Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	383	210
90 Days or More Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 19,556	$ 19,152