GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 883,025		$ 883,025		$ 204,084	$ 204,084
Intangible Assets, Net (Excluding Goodwill)	51,600		51,600		5,300
Finite-Lived Core Deposits, Gross	42,500		42,500		$ 3,300
Finite-Lived Intangible Assets, Amortization Expense	$ 2,400	$ 300	$ 2,600	$ 700
Core Deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets amortization method			accelerated basis
Estimated weighted average life (in years)			9 years 3 months 18 days