ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 9,237		$ 8,592
Accruing Current Balances	4,770,976		4,738,412
Acquired Impaired Loans	20,154	$ 31,247	21,654
Nonaccrual Loans	34,060		36,436
Total Loans	$ 4,834,427	4,577,628	$ 4,805,094
30-59 Days Past Due and Still Accruing, % of Total Loans	0.16%		0.14%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.02%		0.03%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.01%		0.01%
Total Past Due And Still Accruing, % of Total Loans	0.19%		0.18%
Accruing Current Balances, % of Total Loans	98.69%		98.61%
Acquired Impaired Loans, % of Total Loans	0.42%		0.45%
Nonaccrual Loans, % of Total Loans	0.70%		0.76%
% of Total Loans	99.98%		100.00%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 1,142		$ 1,050
Accruing Current Balances	1,467,695		1,440,291
Acquired Impaired Loans	3,870	5,256	4,156
Nonaccrual Loans	16,856		19,057
Total Loans	1,489,563	1,339,867	1,464,554
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,812		3,145
Accruing Current Balances	219,366		225,434
Acquired Impaired Loans	777	866	784
Nonaccrual Loans	4,158		4,124
Total Loans	227,113	259,255	233,487
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,034		1,685
Accruing Current Balances	579,283		554,634
Acquired Impaired Loans	249	263	247
Nonaccrual Loans	2,156		1,927
Total Loans	582,722	464,898	558,493
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	7,550		6,614
30–59 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	403		1,050
30–59 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,446		2,058
30–59 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	532		1,117
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,132		1,517
60–89 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	708		0
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	211		731
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	179		463
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	555		461
Greater Than 90 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	31		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	155		356
Greater Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	323		105
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,529		2,302
Accruing Current Balances	1,069,644		1,067,488
Acquired Impaired Loans	5,147	12,040	5,803
Nonaccrual Loans	4,342		3,654
Total Loans	1,080,662	1,092,292	1,079,247
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	1,529		2,069
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		233
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	696		410
Accruing Current Balances	1,149,408		1,171,701
Acquired Impaired Loans	9,210	10,450	9,724
Nonaccrual Loans	5,110		5,870
Total Loans	1,164,424	1,165,556	1,187,705
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	616		320
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	34		90
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	46		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,024		0
Accruing Current Balances	285,580		278,864
Acquired Impaired Loans	901	2,372	940
Nonaccrual Loans	1,438		1,804
Total Loans	289,943	$ 255,760	281,608
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,024		0
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