SEGMENT REPORTING (Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest income	$ 75,504	$ 45,958
Interest expense	12,613	5,705
Provision for loan losses	1,997	194	$ 2,977
Non-interest income	17,118	12,632
Non-interest Expense	49,641	29,981
Income before income taxes	28,371	22,710
Income tax expense	6,706	7,598
Net income	21,665	15,112
Assets	7,894,918	5,201,164	$ 5,446,675
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(1)	(2)
Interest expense	(1)	(2)
Provision for loan losses	0	0
Non-interest income	(154)	(209)
Non-interest Expense	(154)	(209)
Income before income taxes	0	0
Income tax expense	0	0
Net income	0	0
Assets	(25,246)	(24,039)
Community Banking
Segment Reporting Information [Line Items]
Interest income	75,504	45,957
Interest expense	12,614	5,707
Provision for loan losses	1,997	194
Non-interest income	13,171	9,086
Non-interest Expense	47,031	27,799
Income before income taxes	27,033	21,343
Income tax expense	6,353	7,060
Net income	20,680	14,283
Assets	7,894,964	5,202,560
Insurance
Segment Reporting Information [Line Items]
Interest income	0	1
Interest expense	0	0
Provision for loan losses	0	0
Non-interest income	1,822	1,752
Non-interest Expense	1,379	1,355
Income before income taxes	443	398
Income tax expense	123	161
Net income	320	237
Assets	8,984	8,066
Investment Management
Segment Reporting Information [Line Items]
Interest income	1	2
Interest expense	0	0
Provision for loan losses	0	0
Non-interest income	2,279	2,003
Non-interest Expense	1,385	1,036
Income before income taxes	895	969
Income tax expense	230	377
Net income	665	592
Assets	$ 16,216	$ 14,577