SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended			3 Months Ended		1 Months Ended		1 Months Ended		1 Months Ended	0 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 CBIC and Rockbridge	Mar. 31, 2013 Surety segment Acquisitions prior to 2011	Apr. 30, 2011 Surety segment Contractors Bonding and Insurance Company	Mar. 31, 2013 Surety segment Contractors Bonding and Insurance Company	Apr. 30, 2011 Casualty and Surety segment Contractors Bonding and Insurance Company	Mar. 31, 2013 Casualty and Surety segment Contractors Bonding and Insurance Company	Apr. 30, 2011 Casualty Contractors Bonding and Insurance Company	Nov. 02, 2012 Casualty Rockbridge	Mar. 31, 2013 Casualty Rockbridge
INTANGIBLE ASSETS
Goodwill and intangibles	$ 75,787,000		$ 76,113,000						$ 33,100,000			$ 16,500,000
Goodwill					26,200,000		15,100,000		5,300,000			12,400,000
Goodwill acquired						15,100,000				5,300,000	12,400,000
Indefinite Lived Intangible Assets									8,800,000
Indefinite Lived Intangible Assets Acquired								8,800,000
Impairment of assets	0
Definite-lived intangible assets, net of amortization									3,900,000			4,100,000
Amortization of intangible assets				300,000
Basic EPS, Income (Numerator)
Income available to common shareholders	24,847,000	28,038,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	$ 24,847,000	$ 28,038,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	21,272	21,188
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	21,623	21,528
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 1.17	$ 1.32
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 1.15	$ 1.30
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	351	340