Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Current	$ 3,749,763	$ 3,629,661
30-59 Days Past Due	16,173	17,713
60-89 Days Past Due	5,135	8,789
90 Days or More Past Due	30,311	31,599
Total Past Due	51,619	58,101
Total Loans	3,801,382	3,687,762
Loans held for sale	14,517	21,903
90 Days or More Past Due and Accruing	3,594	5,294
Total loans, current	3,764,280	3,651,564
Total loans	3,815,899	3,709,665
Loans Held for Sale [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans held for sale, current	14,517	21,903
30-59 Days Past Due
60-89 Days Past Due
90 Days or More Past Due
Total Past Due
Loans held for sale	14,517	21,903
90 Days or More Past Due and Accruing
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	193,117	189,072
30-59 Days Past Due	303	1,470
60-89 Days Past Due
90 Days or More Past Due	2,078	2,462
Total Past Due	2,381	3,932
Total Loans	195,498	193,004
90 Days or More Past Due and Accruing	248
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,663,810	1,643,833
30-59 Days Past Due	2,459	5,722
60-89 Days Past Due	2,667	2,224
90 Days or More Past Due	16,649	13,562
Total Past Due	21,775	21,508
Total Loans	1,685,585	1,665,341
90 Days or More Past Due and Accruing		338
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,856,927	1,832,905
30-59 Days Past Due	2,762	7,192
60-89 Days Past Due	2,667	2,224
90 Days or More Past Due	18,727	16,024
Total Past Due	24,156	25,440
Total Loans	1,881,083	1,858,345
90 Days or More Past Due and Accruing	248	338
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	537,925	475,186
30-59 Days Past Due	2,454	283
60-89 Days Past Due	16	412
90 Days or More Past Due	1,676	2,144
Total Past Due	4,146	2,839
Total Loans	542,071	478,025
90 Days or More Past Due and Accruing	114	98
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	815,913	774,006
30-59 Days Past Due	5,956	4,231
60-89 Days Past Due	1,581	4,833
90 Days or More Past Due	7,912	10,632
Total Past Due	15,449	19,696
Total Loans	831,362	793,702
90 Days or More Past Due and Accruing	2,135	3,199
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	277,126	274,235
30-59 Days Past Due	1,603	1,352
60-89 Days Past Due	172	197
90 Days or More Past Due	1,467	1,442
Total Past Due	3,242	2,991
Total Loans	280,368	277,226
90 Days or More Past Due and Accruing	765	722
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	261,872	273,329
30-59 Days Past Due	3,398	4,655
60-89 Days Past Due	699	1,123
90 Days or More Past Due	529	1,357
Total Past Due	4,626	7,135
Total Loans	266,498	280,464
90 Days or More Past Due and Accruing	332	937
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	12,346	11,724
30-59 Days Past Due	1,905	591
60-89 Days Past Due	2,420	1,747
90 Days or More Past Due	26,398	25,310
Total Past Due	30,723	27,648
Total Loans	43,069	39,372
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	17,759	21,665
30-59 Days Past Due	1,052	794
60-89 Days Past Due	139	827
90 Days or More Past Due	319	995
Total Past Due	1,510	2,616
Total Loans	$ 19,269	$ 24,281