SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets
Goodwill			$ 49,657	$ 51,270
State insurance licenses			7,500	7,500
Definite-lived intangibles, net of accumulated amortization of $5,678 at 12/31/17 and $5,546 at 12/31/16			2,145	5,601
Accumulated amortization of definite-lived intangibles			5,678	5,546
Total intangibles			9,645	13,101
Goodwill and intangibles			59,302	64,371
Goodwill, Impairment Loss			3,400
Amortization of intangible assets			700	900	$ 900
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months			500
Finite-Lived Intangible Assets, Amortization Expense, Year Two			500
Finite-Lived Intangible Assets, Amortization Expense, Year Three			500
Finite-Lived Intangible Assets, Amortization Expense, Year Four			200
Finite-Lived Intangible Assets, Amortization Expense, Year Five			100
Indefinite-lived Intangible Assets, Written off Related to Sale of Business Unit			1,000
Energy Surety
Goodwill and Intangible Assets
Goodwill			25,706	25,706
Miscellaneous and Contract Surety
Goodwill and Intangible Assets
Goodwill			15,110	15,110
Small commercial
Goodwill and Intangible Assets
Goodwill			5,246	5,246
Medical professional liability
Goodwill and Intangible Assets
Goodwill			3,595	5,208
Casualty segment | Medical professional liability
Goodwill and Intangible Assets
Goodwill, Impaired, Accumulated Impairment Loss			$ 8,800	$ 7,200
Goodwill and Intangible Asset Impairment	$ 3,400
Goodwill, Impairment Loss	1,600	$ 7,200
Impairment of Intangible Assets, Finite-lived	$ 1,800