SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets and EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
ADOPTED ACCOUNTING STANDARDS
Net Cash Provided by Operating Activities					$ 145,933	$ 123,000
Net Cash Used in Financing Activities					(22,998)	(17,686)
ExpectedValueOfAssetAndLiabilityAdditionFromAdoptionOfASU2016-02	$ 30,000				$ 30,000
PercentageOfFixedIncomePortfolioRatedAOrBetter	82.80%				82.80%
Goodwill	$ 49,657				$ 49,657		$ 51,270
State insurance licenses	7,500				7,500		7,500
Definite-lived intangibles, net of accumulated amortization of $4,061 at 9/30/17 and $5,546 at 12/31/16	2,270				2,270		5,601
Total intangibles	9,770				9,770		13,101
Goodwill and intangibles	59,427				59,427		64,371
Accumulated amortization of definite-lived intangibles	4,061				4,061		5,546
Amortization of intangible assets	200		$ 200		600	700
Finite-Lived Intangible Assets Disposed Of					1,000
Basic EPS, Income (Numerator)
Income available to common shareholders	1,734		22,263		47,770	82,733
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 1,734		$ 22,263		$ 47,770	$ 82,733
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,058		43,843		44,008	43,721
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	457		649		509	695
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,515		44,492		44,517	44,416
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.04		$ 0.51		$ 1.09	$ 1.89
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.04		$ 0.50		$ 1.07	$ 1.86
Accounting Standards Update 2016-09
ADOPTED ACCOUNTING STANDARDS
Reduction to income tax expense					$ 3,700
Net Cash Provided by Operating Activities					3,700
Net Cash Used in Financing Activities					(3,700)
Prior period adjustment					0
Energy Surety
ADOPTED ACCOUNTING STANDARDS
Goodwill	$ 25,706				25,706		25,706
Miscellaneous and Contract Surety
ADOPTED ACCOUNTING STANDARDS
Goodwill	15,110				15,110		15,110
Small commercial
ADOPTED ACCOUNTING STANDARDS
Goodwill	5,246				5,246		5,246
Medical professional liability
ADOPTED ACCOUNTING STANDARDS
Goodwill	3,595				3,595		5,208
Goodwill and Intangible Asset Impairment		$ 3,400
Goodwill, Impairment Loss		1,600		$ 7,200
Goodwill, Impaired, Accumulated Impairment Loss	$ 8,800				$ 8,800		$ 7,200
Impairment of Intangible Assets, Finite-lived		$ 1,800