SEGMENT REPORTING (Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest income	$ 84,374	$ 49,589	$ 238,475	$ 144,123
Interest expense	16,783	6,892	44,175	18,847
Provision for loan losses	1,890	934	5,620	2,450	$ 2,977
Non-interest income	15,033	12,746	47,019	38,949
Non-interest Expense	42,393	31,191	137,116	94,040
Income before income taxes	38,341	23,318	98,583	67,735
Income tax expense	9,107	8,229	23,285	22,793
Net income	29,234	15,089	75,298	44,942
Assets	8,034,565	5,334,788	8,034,565	5,334,788	$ 5,446,675
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(3)	(3)	(6)	(7)
Interest expense	(3)	(3)	(6)	(7)
Provision for loan losses	0	0	0	0
Non-interest income	(154)	(209)	(461)	(628)
Non-interest Expense	(154)	(209)	(461)	(628)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	(29,140)	(18,057)	(29,140)	(18,057)
Community Banking
Segment Reporting Information [Line Items]
Interest income	84,374	49,590	238,474	144,123
Interest expense	16,786	6,895	44,181	18,854
Provision for loan losses	1,890	934	5,620	2,450
Non-interest income	10,707	8,915	35,130	28,545
Non-interest Expense	39,372	28,634	128,705	87,035
Income before income taxes	37,033	22,042	95,098	64,329
Income tax expense	8,755	7,725	22,357	21,454
Net income	28,278	14,317	72,741	42,875
Assets	8,038,263	5,333,539	8,038,263	5,333,539
Insurance
Segment Reporting Information [Line Items]
Interest income	1	0	2	1
Interest expense	0	0	0	0
Provision for loan losses	0	0	0	0
Non-interest income	2,018	1,948	5,019	4,923
Non-interest Expense	1,564	1,559	4,245	4,238
Income before income taxes	455	389	776	686
Income tax expense	127	158	218	279
Net income	328	231	558	407
Assets	9,473	7,499	9,473	7,499
Investment Management
Segment Reporting Information [Line Items]
Interest income	2	2	5	6
Interest expense	0	0	0	0
Provision for loan losses	0	0	0	0
Non-interest income	2,462	2,092	7,331	6,109
Non-interest Expense	1,611	1,207	4,627	3,395
Income before income taxes	853	887	2,709	2,720
Income tax expense	225	346	710	1,060
Net income	628	541	1,999	1,660
Assets	$ 15,969	$ 11,807	$ 15,969	$ 11,807