SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
INTANGIBLE ASSETS
Goodwill and intangibles	$ 72,249		$ 72,249		$ 72,695
Basic EPS, Income (Numerator)
Income available to common shareholders	37,185	$ 35,725	67,783	$ 64,694
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 37,185	$ 35,725	$ 67,783	$ 64,694
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	43,210	43,001	43,176	42,993
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	809	687	832	676
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	44,019	43,688	44,008	43,669
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.86	$ 0.83	$ 1.57	$ 1.50
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.84	$ 0.82	$ 1.54	$ 1.48
CBIC and Rockbridge
INTANGIBLE ASSETS
Amortization of intangible assets	$ 200		$ 400
Surety segment | Acquisitions prior to 2011
INTANGIBLE ASSETS
Goodwill and intangibles	26,200		26,200
Goodwill	25,600		25,600
Indefinite-Lived Intangible Assets (Excluding Goodwill)	600		600
Surety segment | Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill	15,100		15,100
Casualty and Surety segment | Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill and intangibles	30,300		30,300
Indefinite-Lived Intangible Assets (Excluding Goodwill)	7,500		7,500
Definite-lived intangible assets, net of amortization	2,400		2,400
Casualty segment | Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill	5,300		5,300
Casualty segment | Rockbridge
INTANGIBLE ASSETS
Goodwill and intangibles	15,700		15,700
Goodwill	12,400		12,400
Definite-lived intangible assets, net of amortization	$ 3,300		$ 3,300