SEGMENT REPORTING (Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest income	$ 78,597	$ 48,576	$ 154,101	$ 94,534
Interest expense	14,779	6,250	27,392	11,955
Provision for loan losses	1,733	1,322	3,730	1,516	$ 2,977
Non-interest income	14,868	13,571	31,986	26,203
Non-interest Expense	45,082	32,868	94,723	62,849
Income before income taxes	31,871	21,707	60,242	44,417
Income tax expense	7,472	6,966	14,178	14,564
Net income	24,399	14,741	46,064	29,853
Assets	8,152,600	5,270,521	8,152,600	5,270,521	$ 5,446,675
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(2)	(2)	(3)	(4)
Interest expense	(2)	(2)	(3)	(4)
Provision for loan losses	0	0	0	0
Non-interest income	(153)	(210)	(307)	(419)
Non-interest Expense	(153)	(210)	(307)	(419)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	(26,805)	(25,024)	(26,805)	(25,024)
Community Banking
Segment Reporting Information [Line Items]
Interest income	78,596	48,576	154,100	94,533
Interest expense	14,781	6,252	27,395	11,959
Provision for loan losses	1,733	1,322	3,730	1,516
Non-interest income	11,252	10,544	24,423	19,630
Non-interest Expense	42,302	30,602	89,333	58,401
Income before income taxes	31,032	20,944	58,065	42,287
Income tax expense	7,249	6,669	13,602	13,729
Net income	23,783	14,275	44,463	28,558
Assets	8,154,957	5,273,823	8,154,957	5,273,823
Insurance
Segment Reporting Information [Line Items]
Interest income	1	0	1	1
Interest expense	0	0	0	0
Provision for loan losses	0	0	0	0
Non-interest income	1,179	1,223	3,001	2,975
Non-interest Expense	1,302	1,324	2,681	2,679
Income before income taxes	(122)	(101)	321	297
Income tax expense	(32)	(40)	91	121
Net income	(90)	(61)	230	176
Assets	9,041	8,046	9,041	8,046
Investment Management
Segment Reporting Information [Line Items]
Interest income	2	2	3	4
Interest expense	0	0	0	0
Provision for loan losses	0	0	0	0
Non-interest income	2,590	2,014	4,869	4,017
Non-interest Expense	1,631	1,152	3,016	2,188
Income before income taxes	961	864	1,856	1,833
Income tax expense	255	337	485	714
Net income	706	527	1,371	1,119
Assets	$ 15,407	$ 13,676	$ 15,407	$ 13,676