SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Information about reportable segments
Net interest income	$ 84,395	$ 68,463	$ 229,698	$ 203,650
Provision for loan losses	2,314	1,790	5,603	5,436
Noninterest revenues	52,941	39,952	148,485	117,005
Amortization of intangible assets	4,949	1,359	11,980	4,204
Acquisition expenses	580	2	25,192	342
Other operating expenses	78,247	64,865	223,058	195,705
Income before income taxes	51,246	40,399	112,350	114,968
Assets	10,850,218	8,727,746	10,850,218	8,727,746	$ 8,666,437
Goodwill	731,505	465,142	731,505	465,142	$ 465,142
Eliminations [Member]
Information about reportable segments
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(684)	(623)	(2,035)	(1,784)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(684)	(623)	(2,035)	(1,784)
Income before income taxes	0	0	0	0
Assets	(67,461)	(39,415)	(67,461)	(39,415)
Goodwill	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	84,227	68,375	229,233	203,394
Provision for loan losses	2,314	1,790	5,603	5,436
Noninterest revenues	20,120	17,756	54,049	49,663
Amortization of intangible assets	1,796	665	3,520	2,075
Acquisition expenses	534	2	23,784	101
Other operating expenses	56,926	47,736	160,311	143,781
Income before income taxes	42,777	35,938	90,064	101,664
Assets	10,613,065	8,658,308	10,613,065	8,658,308
Goodwill	629,153	440,870	629,153	440,870
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	104	39	276	117
Provision for loan losses	0	0	0	0
Noninterest revenues	21,207	11,680	59,961	36,137
Amortization of intangible assets	2,323	96	6,256	326
Acquisition expenses	11	0	1,190	0
Other operating expenses	12,788	9,020	37,225	27,899
Income before income taxes	6,189	2,603	15,566	8,029
Assets	226,812	36,706	226,812	36,706
Goodwill	84,448	8,019	84,448	8,019
All Other [Member]
Information about reportable segments
Net interest income	64	49	189	139
Provision for loan losses	0	0	0	0
Noninterest revenues	12,298	11,139	36,510	32,989
Amortization of intangible assets	830	598	2,204	1,803
Acquisition expenses	35	0	218	241
Other operating expenses	9,217	8,732	27,557	25,809
Income before income taxes	2,280	1,858	6,720	5,275
Assets	77,802	72,147	77,802	72,147
Goodwill	$ 17,904	$ 16,253	$ 17,904	$ 16,253