INCOME TAXES - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Deferred tax assets:
Discounting of net reserves for losses and loss expenses	$ 27,804	$ 37,440
Unearned premiums	25,188	40,447
Net unrealized investments losses	0	11,438
Operating and capital loss carryforwards	53,095	83,850
Accruals not currently deductible	31,560	32,589
Tax credits	29,929	8,672
Other deferred tax assets	15,047	9,195
Deferred tax assets before valuation allowance	182,623	223,631
Valuation allowance	(16,157)	(18,955)
Deferred tax assets net of valuation allowance	166,466	204,676
Deferred tax liabilities:
Deferred acquisition costs	(24,249)	(39,745)
Net unrealized investments gains	(8,033)	0
Amortization of VOBA, intangible assets and goodwill	(85,296)	(49,097)
Equalization reserves	(23,274)	(22,069)
Other deferred tax liabilities	(21,176)	(53,990)
Deferred tax liabilities	(162,028)	(164,901)
Net deferred tax assets	4,438	39,775	$ 103,313
Deferred tax asset, provisional income tax expense	41,600
Deferred tax liabilities, business acquisition		103,800
Lloyd's
Deferred tax assets:
Operating and capital loss carryforwards	$ 19,000	$ 68,000