LEASES - Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leases
Expense associated with operating leases			$ 6,900	$ 6,800	$ 6,400
Operating Lease Cost	$ 1,423	$ 4,349
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	1,438	4,234
Right-of-use assets obtained in exchange for new operating lease liabilities	427	1,378
Reduction to right-of-use asset resulting from reduction to lease liability		1,279
Operating lease right-of-use assets	$ 23,604	$ 23,604
Operating lease, right-of-use asset, Statement of Financial Position	us-gaap:OtherAssets	us-gaap:OtherAssets
Operating lease liabilities	$ 25,787	$ 25,787
Operating lease liability, Statement of Financial Position	us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Weighted-average remaining lease term - operating leases	4 years 10 months 28 days	4 years 10 months 28 days
Weighted-average discount rate - operating leases	2.33%	2.33%
Minimum
Leases
Remaining lease terms - operating leases		1 year
Maximum
Leases
Remaining lease terms - operating leases		16 years