SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets and EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
ADOPTED ACCOUNTING STANDARDS
Increase to operating cash flows	$ 11,265	$ 21,266
Decrease to financing cash flows	7,294	4,431
Goodwill	51,270		$ 51,270
State insurance licenses	7,500		7,500
Definite-lived intangibles, net of accumulated amortization of $3,606 at 3/31/17 and $5,546 at 12/31/16	5,395		5,601
Total intangibles	12,895		13,101
Goodwill and intangibles	64,165		64,371
Accumulated amortization of definite-lived intangibles	3,606		5,546
Amortization of intangible assets	200	200
Basic EPS, Income (Numerator)
Income available to common shareholders	19,828	31,393
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 19,828	$ 31,393
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	43,961	43,597
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	541	764
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,502	44,361
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.45	$ 0.72
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.45	$ 0.71
Accounting Standards Update 2016-09
ADOPTED ACCOUNTING STANDARDS
Reduction to income tax expense	$ 400
Increase to operating cash flows	400
Decrease to financing cash flows	400
Prior period adjustment	0
Energy Surety
ADOPTED ACCOUNTING STANDARDS
Goodwill	25,706		25,706
Miscellaneous and Contract Surety
ADOPTED ACCOUNTING STANDARDS
Goodwill	15,110		15,110
Small commercial
ADOPTED ACCOUNTING STANDARDS
Goodwill	5,246		5,246
Medical professional liability
ADOPTED ACCOUNTING STANDARDS
Goodwill	5,208		5,208
Goodwill, Impaired, Accumulated Impairment Loss	$ 7,200		$ 7,200