Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 5,081		$ 7,730
Accruing Current Balances	4,531,161		4,404,996
Acquired Impaired Loans	31,147	$ 27,072	33,614
Nonaccrual Loans	38,382		22,876
Total Loans	$ 4,605,771	3,847,515	$ 4,469,216
30-59 Days Past Due and Still Accruing, % of Total Loans	0.09%		0.12%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.02%		0.04%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.00%		0.01%
Total Past Due And Still Accruing, % of Total Loans	0.11%		0.17%
Accruing Current Balances, % of Total Loans	98.38%		98.57%
Acquired Impaired Loans, % of Total Loans	0.68%		0.75%
Nonaccrual Loans, % of Total Loans	0.83%		0.51%
% of Total Loans	100.00%		100.00%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 907		$ 1,785
Accruing Current Balances	1,302,051		1,277,748
Acquired Impaired Loans	6,002	1,711	6,183
Nonaccrual Loans	14,704		2,015
Total Loans	1,323,664	1,108,610	1,287,731
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,759		3,967
Accruing Current Balances	244,031		257,234
Acquired Impaired Loans	852	946	860
Nonaccrual Loans	4,960		4,967
Total Loans	252,602	236,657	267,028
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,178		1,023
Accruing Current Balances	487,897		444,642
Acquired Impaired Loans	246	4	369
Nonaccrual Loans	3,496		3,995
Total Loans	492,817	374,634	450,029
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	4,147		5,490
30–59 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	901		1,507
30–59 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,411		3,176
30–59 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	598		392
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	842		1,802
60–89 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	6		278
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	348		638
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	488		346
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	92		438
Greater Than 90 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Greater Than 90 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		153
Greater Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	92		285
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		656
Accruing Current Balances	1,094,605		1,063,306
Acquired Impaired Loans	11,839	9,757	12,122
Nonaccrual Loans	3,432		2,078
Total Loans	1,109,876	938,330	1,078,162
Owner-occupied commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		116
Owner-occupied commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		540
Owner-occupied commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	237		167
Accruing Current Balances	1,125,854		1,143,180
Acquired Impaired Loans	10,187	10,560	10,386
Nonaccrual Loans	10,728		9,821
Total Loans	1,147,006	990,245	1,163,554
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	237		167
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		132
Accruing Current Balances	276,723		218,886
Acquired Impaired Loans	2,021	4,094	3,694
Nonaccrual Loans	1,062		0
Total Loans	279,806	$ 199,039	222,712
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		132
Construction | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 0		$ 0