ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 8,925		$ 8,592
Accruing Current Balances	4,856,171		4,738,412
Acquired Impaired Loans	17,493	$ 24,582	21,654
Nonaccrual Loans	36,688		36,436
Total Loans	$ 4,919,277	4,696,231	$ 4,805,094
30-59 Days Past Due and Still Accruing, % of Total Loans	0.13%		0.14%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.05%		0.03%
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	0.18%		0.18%
Accruing Current Balances, % of Total Loans	98.72%		98.61%
Acquired Impaired Loans, % of Total Loans	0.36%		0.45%
Nonaccrual Loans, % of Total Loans	0.75%		0.76%
% of Total Loans	100.00%		100.00%
Reverse mortgage, fair value	$ 16,600	21,400	$ 19,800
Acquired nonimpaired loans	465,438	596,466	565,500
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,470		1,050
Accruing Current Balances	1,484,007		1,440,291
Acquired Impaired Loans	2,070	5,235	4,156
Nonaccrual Loans	19,128		19,057
Total Loans	1,506,675	1,388,094	1,464,554
Acquired nonimpaired loans	97,552	120,987
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	3,239		3,145
Accruing Current Balances	199,456		225,434
Acquired Impaired Loans	766	788	784
Nonaccrual Loans	3,265		4,124
Total Loans	206,726	240,717	233,487
Acquired nonimpaired loans	60,407	77,154
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	971		1,685
Accruing Current Balances	650,226		554,634
Acquired Impaired Loans	153	243	247
Nonaccrual Loans	2,354		1,927
Total Loans	653,704	520,276	558,493
Acquired nonimpaired loans	24,568	40,136
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	6,472		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	1,324		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	2,336		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	612		1,117
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,242		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	144		0
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	879		731
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	174		463
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	211		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	2		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	24		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	185		105
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,308		2,302
Accruing Current Balances	1,075,451		1,067,488
Acquired Impaired Loans	4,816	7,401	5,803
Nonaccrual Loans	2,829		3,654
Total Loans	1,085,404	1,096,753	1,079,247
Acquired nonimpaired loans	119,403	144,710
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	1,526		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	782		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	937		410
Accruing Current Balances	1,117,184		1,171,701
Acquired Impaired Loans	8,951	9,969	9,724
Nonaccrual Loans	6,209		5,870
Total Loans	1,133,281	1,157,074	1,187,705
Acquired nonimpaired loans	156,483	194,394
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	674		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	263		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	0		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Accruing Current Balances	329,847		278,864
Acquired Impaired Loans	737	946	940
Nonaccrual Loans	2,903		1,804
Total Loans	333,487	293,317	281,608
Acquired nonimpaired loans	7,025	$ 19,085
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		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