BUSINESS COMBINATIONS - Assets acquired and liabilities assumed assets acquired and liabilities assumed (Details) - USD ($) $ in Thousands	Oct. 02, 2017	Dec. 31, 2018	Dec. 31, 2017
Fair value of identifiable intangible assets:
Goodwill		$ 102,003	$ 102,003
Novae
Business Acquisition [Line Items]
Total purchase price paid	$ 616,926
Assets
Investments	1,733,611
Cash and cash equivalents	191,337
Insurance and reinsurance premium balances receivable	472,180
Reinsurance recoverable on unpaid and paid losses	787,907
Prepaid reinsurance premiums	197,907
Other assets	42,696
Total assets acquired	3,425,638
Liabilities
Reserve for losses and loss expenses	2,125,634
Unearned premiums	717,442
Insurance and reinsurance balances payable	273,405
Notes payable	101,846
Other liabilities	124,585
Total liabilities assumed	3,342,912
Fair value of identifiable intangible assets:
Value of business acquired - definite lived intangible asset	256,942
Finite lived other intangible assets	128,463
Indefinite lived intangible assets	94,748
Goodwill	$ 54,047