GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details) - USD ($) $ in Thousands			3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 30, 2019	Apr. 01, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill resulting from business combinations	$ 58,000	$ 675,643	$ 58,000		$ 57,438	$ 676,949	$ 676,167
Goodwill			937,689		937,689		$ 880,251	$ 204,084
Other Depreciation and Amortization			2,400		6,500	5,100
Intangible assets amortization			$ 2,432	$ 2,486	6,521	$ 5,130
Customer lists
Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired					$ 39,400
Intangible assets amortization method					straight-line basis
Estimated weighted average life (in years)					11 years
Core deposits
Finite-Lived Intangible Assets [Line Items]
Intangible assets amortization method					accelerated basis
Estimated weighted average life (in years)					8 years 2 months 12 days