SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Intangible Assets and EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
INTANGIBLE ASSETS
Goodwill and intangibles	$ 72,232		$ 71,294
Goodwill	58,496		58,496
State insurace licenses	7,500		7,500
Definite-lived intangibles, net of accumulated amortization of $4,911 at 3/31/16 and $4,678 at 12/31/15	6,236		5,298
Total intangibles	13,736		12,798
Finite-Lived Intangible Assets, Accumulated Amortization	4,911		4,678
Finite-lived Intangible Assets Acquired	1,200
Basic EPS, Income (Numerator)
Income available to common shareholders	31,393	$ 30,598
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 31,393	$ 30,598
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	43,597	43,141
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	764	851
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,361	43,992
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.72	$ 0.71
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.71	$ 0.70
CBIC and Rockbridge
INTANGIBLE ASSETS
Amortization of intangible assets	$ 200	$ 200
Energy Surety | Surety segment
INTANGIBLE ASSETS
Goodwill	25,706		25,706
Miscellaneous and Contract Surety | Surety segment
INTANGIBLE ASSETS
Goodwill	15,110		15,110
P&C package business | Casualty segment
INTANGIBLE ASSETS
Goodwill	5,246		5,246
Medical professional liability | Casualty segment
INTANGIBLE ASSETS
Goodwill	$ 12,434		$ 12,434