SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets and EPS (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 46,062,000				$ 46,062,000		$ 46,062,000
State insurance licenses	7,500,000				7,500,000		7,500,000
Definite-lived intangibles, net of accumulated amortization of $3,368 at 9/30/19 and $3,062 at 12/31/18	666,000				666,000		972,000
Total intangibles	8,166,000				8,166,000		8,472,000
Goodwill and intangibles	54,228,000				54,228,000		54,534,000
Accumulated amortization of definite-lived intangibles	3,368,000				3,368,000		3,062,000
Amortization of intangible assets	100,000		$ 100,000		300,000	$ 300,000
Basic EPS, Income (Numerator)
Income available to common shareholders	32,324,000		39,372,000		138,264,000	84,839,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 32,324,000		$ 39,372,000		$ 138,264,000	$ 84,839,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,823		44,400		44,689	44,311
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	526		540		503	449
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	45,349		44,940		45,192	44,760
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.72		$ 0.89		$ 3.09	$ 1.91
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.71		$ 0.88		$ 3.06	$ 1.90
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
Total intangibles				0
Goodwill and Intangible Asset Impairment				4,400,000
Goodwill, Impairment Loss				3,600,000
Impairment of Intangible Assets, Finite-lived				$ 800,000