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, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 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 $3,164 at 3/31/19 and $3,062 at 12/31/18	768,000			768,000		972,000
Total intangibles	8,268,000			8,268,000		8,472,000
Goodwill and intangibles	54,330,000			54,330,000		54,534,000
Accumulated amortization of definite-lived intangibles	3,266,000			3,266,000		3,062,000
Amortization of intangible assets	100,000	$ 100,000		200,000	$ 200,000
Basic EPS, Income (Numerator)
Income available to common shareholders	40,467,000	33,251,000		105,940,000	45,467,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 40,467,000	$ 33,251,000		$ 105,940,000	$ 45,467,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,704	44,310		44,620	44,266
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	515	432		436	587
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	45,219	44,742		45,056	44,853
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.91	$ 0.75		$ 2.37	$ 1.03
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.89	$ 0.74		$ 2.35	$ 1.01
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