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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Business Acquisition
Goodwill	$ 46,062,000		$ 46,062,000		$ 46,062,000
Indefinite-lived intangibles - state insurance licenses	7,500,000		7,500,000		7,500,000
Definite-lived intangibles, net of accumulated amortization of $3,674 at 6/30/20 and $3,470 at 12/31/19	361,000		361,000		565,000
Total intangibles	7,861,000		7,861,000		8,065,000
Goodwill and intangibles	53,923,000		53,923,000		54,127,000
Accumulated amortization of definite-lived intangibles	3,674,000		3,674,000		3,470,000
Amortization of intangible assets	100,000	$ 100,000	200,000	$ 200,000
Basic EPS, Income (Numerator)
Income available to common shareholders	92,166,000	40,467,000	30,899,000	105,940,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 92,166,000	$ 40,467,000	$ 30,899,000	$ 105,940,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,951	44,704	44,936	44,620
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	323	515	375	436
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	45,274	45,219	45,311	45,056
Earnings Per Share, Diluted, Other Disclosures
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	94		335	254
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 2.05	$ 0.91	$ 0.69	$ 2.37
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 2.04	$ 0.89	$ 0.68	$ 2.35
Contractors Bonding and Insurance Company
Business Acquisition
Impairment of indefinite-lived intangible assets			$ 0
Energy Surety
Business Acquisition
Goodwill	$ 25,706,000		25,706,000		25,706,000
Goodwill, Impairment Loss			0
Miscellaneous and Contract Surety
Business Acquisition
Goodwill	15,110,000		15,110,000		15,110,000
Goodwill, Impairment Loss			0
Small commercial
Business Acquisition
Goodwill	$ 5,246,000		5,246,000		$ 5,246,000
Goodwill, Impairment Loss			$ 0