Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Current	$ 4,893,737	$ 4,042,825
Total Past Due	39,908	43,941
Total Loans	4,933,645	4,086,766
90 Days or More Past Due and Accruing	2,471	2,288
Loans held for sale, current	11,160	5,865
Loans held for sale	11,160	5,865
Total loans, current	4,904,897	4,048,690
Total loans	4,944,805	4,092,631
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,891	8,500
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,643	9,976
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	26,374	25,465
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	307,522	261,356
Total Past Due	1,363	1,287
Total Loans	308,885	262,643
90 Days or More Past Due and Accruing		71
Commercial Real Estate - Land and Construction [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	252	20
Commercial Real Estate - Land and Construction [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,111	1,267
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,870,393	1,665,363
Total Past Due	14,835	17,454
Total Loans	1,885,228	1,682,817
90 Days or More Past Due and Accruing	180
Commercial Real Estate - Improved Property [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,304	961
Commercial Real Estate - Improved Property [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	797	4,772
Commercial Real Estate - Improved Property [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	12,734	11,721
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,177,915	1,926,719
Total Past Due	16,198	18,741
Total Loans	2,194,113	1,945,460
90 Days or More Past Due and Accruing	180	71
Commercial Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,556	981
Commercial Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	797	4,772
Commercial Real Estate [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	13,845	12,988
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	729,815	634,482
Total Past Due	3,663	3,928
Total Loans	733,478	638,410
90 Days or More Past Due and Accruing	3	22
Commercial and Industrial [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	356	1,834
Commercial and Industrial [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	896	240
Commercial and Industrial [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,411	1,854
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,229,198	915,968
Total Past Due	12,272	12,802
Total Loans	1,241,470	928,770
90 Days or More Past Due and Accruing	1,838	1,306
Residential Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,014	1,237
Residential Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,322	3,384
Residential Real Estate [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	7,936	8,181
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	375,663	325,291
Total Past Due	4,077	4,740
Total Loans	379,740	330,031
90 Days or More Past Due and Accruing	228	570
Home Equity [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,212	1,877
Home Equity [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,010	895
Home Equity [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,855	1,968
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	381,146	240,365
Total Past Due	3,698	3,730
Total Loans	384,844	244,095
90 Days or More Past Due and Accruing	222	319
Consumer [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,753	2,571
Consumer [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	618	685
Consumer [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	327	474
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	21,964	7,562
Total Past Due	26,240	31,256
Total Loans	48,204	38,818
Non-Accrual Loans [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,030	2,884
Non-Accrual Loans [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,641	5,552
Non-Accrual Loans [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	23,569	22,820
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	12,081	11,016
Total Past Due	877	1,050
Total Loans	12,958	12,066
TDRs Accruing Interest [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	109	151
TDRs Accruing Interest [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	434	542
TDRs Accruing Interest [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	334	357
Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	34,045	18,578
Total Past Due	27,117	32,306
Total Loans	61,162	50,884
Total Impaired [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,139	3,035
Total Impaired [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,075	6,094
Total Impaired [Member] | 90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 23,903	$ 23,177