OTHER COMPREHENSIVE INCOME - RECLASSIFICATION (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant amounts reclassified out of each component of AOCI
Noninterest income	$ 19,655	$ 12,811	$ 15,125	$ 27,417	$ 10,119	$ 11,465	$ 14,296	$ 27,545	$ 75,008	$ 63,425	$ 58,414
Interest benefit (expense) on deposits									(29,135)	(17,017)	(9,802)
Interest benefit (expense) on FHLB advances									(12,791)	(10,473)	(8,860)
Total interest benefit (expense)	(10,132)	(12,573)	(11,718)	(10,334)	(8,626)	(8,057)	(7,272)	(6,168)	(44,757)	(30,123)	(20,258)
Income tax (benefit) expense	(6,533)	(4,325)	(3,176)	(7,460)	(3,022)	(1,798)	(4,150)	(7,441)	(21,494)	(16,411)	(32,754)
NET INCOME	$ 25,768	$ 18,408	$ 18,007	$ 29,516	$ 17,306	$ 17,411	$ 15,666	$ 27,469	91,699	77,852	45,632
Reclassification out of Accumulated Other Comprehensive Income
Significant amounts reclassified out of each component of AOCI
NET INCOME									15	(22)	(230)
Unrealized gain (loss) on AFS debt securities | Reclassification out of Accumulated Other Comprehensive Income
Significant amounts reclassified out of each component of AOCI
Noninterest income											(136)
Income tax (benefit) expense											48
NET INCOME											(88)
Unrealized gain (loss) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Significant amounts reclassified out of each component of AOCI
Interest benefit (expense) on deposits									10	(18)	(109)
Interest benefit (expense) on FHLB advances									10	(10)	(110)
Total interest benefit (expense)									20	(28)	(219)
Income tax (benefit) expense									(5)	6	77
NET INCOME									$ 15	$ (22)	$ (142)