SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets and EPS (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 46,062,000			$ 46,062,000		$ 49,657,000
State insurance licenses	7,500,000			7,500,000		7,500,000
Definite-lived intangibles, net of accumulated amortization of $2,862 at 6/30/18 and $5,678 at 12/31/17	1,131,000			1,131,000		2,145,000
Total intangibles	8,631,000			8,631,000		9,645,000
Goodwill and intangibles	54,693,000			54,693,000		59,302,000
Accumulated amortization of definite-lived intangibles	2,862,000			2,862,000		5,678,000
Amortization of intangible assets	100,000		$ 200,000	200,000	$ 400,000
Basic EPS, Income (Numerator)
Income available to common shareholders	33,251,000		26,208,000	45,467,000	46,036,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 33,251,000		$ 26,208,000	$ 45,467,000	$ 46,036,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,310		44,005	44,266	43,983
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	432		514	587	534
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,742		44,519	44,853	44,517
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.75		$ 0.60	$ 1.03	$ 1.05
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.74		$ 0.59	$ 1.01	$ 1.03
Contractors Bonding and Insurance Company
Business Acquisition [Line Items]
Impairment of indefinite-lived intangible assets	$ 0
Energy Surety
Business Acquisition [Line Items]
Goodwill	25,706,000			$ 25,706,000		25,706,000
Goodwill, Impairment Loss	0
Miscellaneous and Contract Surety
Business Acquisition [Line Items]
Goodwill	15,110,000			15,110,000		15,110,000
Goodwill, Impairment Loss	0
Small commercial
Business Acquisition [Line Items]
Goodwill	5,246,000			5,246,000		5,246,000
Goodwill, Impairment Loss	0
Medical professional liability
Business Acquisition [Line Items]
Goodwill	0	$ 0		0		3,595,000
Goodwill and Intangible Asset Impairment		4,400,000	$ 3,400,000
Goodwill, Impairment Loss		3,600,000	1,600,000
Goodwill, Impaired, Accumulated Impairment Loss	$ 12,400,000			$ 12,400,000		$ 8,800,000
Impairment of Intangible Assets, Finite-lived		$ 800,000	$ 1,800,000