SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill and Intangible Assets
Goodwill		$ 51,270	$ 58,496
State insurance licenses		7,500	7,500
Definite-lived intangibles, net of accumulated amortization of $5,546 at 12/31/16 and $4,678 at 12/31/15		5,601	5,298
Accumulated amortization of definite-lived intangibles		5,546	4,678
Total intangibles		13,101	12,798
Goodwill and intangibles		64,371	71,294
Goodwill, Impairment Loss		7,200
Amortization of intangible assets		900	900	$ 900
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		800
Finite-Lived Intangible Assets, Amortization Expense, Year Two		800
Finite-Lived Intangible Assets, Amortization Expense, Year Three		700
Finite-Lived Intangible Assets, Amortization Expense, Year Four		700
Finite-Lived Intangible Assets, Amortization Expense, Year Five		700
Contractors Bonding and Insurance Company
Goodwill and Intangible Assets
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)				$ 1,300
Energy Surety
Goodwill and Intangible Assets
Goodwill		25,706	25,706
Miscellaneous and Contract Surety
Goodwill and Intangible Assets
Goodwill		15,110	15,110
P&C package business
Goodwill and Intangible Assets
Goodwill		5,246	5,246
Medical professional liability
Goodwill and Intangible Assets
Goodwill		5,208	$ 12,434
Casualty segment | Medical professional liability
Goodwill and Intangible Assets
Goodwill, Impaired, Accumulated Impairment Loss		7,200
Goodwill, Impairment Loss	$ 7,200
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability		$ 1,500