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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
INTANGIBLE ASSETS
Goodwill and intangibles	$ 72,472,000		$ 72,695,000
Basic EPS, Income (Numerator)
Income available to common shareholders	30,598,000	28,969,000
Diluted EPS, Income (Numerator)
Income available to common shareholders	30,598,000	28,969,000
Basic EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	43,141	42,985
Effect of Dilutive Securities, Shares (Denominator)
Stock options (in shares)	851	689
Diluted EPS, Weighted Average Shares (Denominator)
Number of shares outstanding	43,992	43,674
Basic EPS, Per Share Amount
Basic net earnings per share (in dollars per share)	$ 0.71us-gaap_EarningsPerShareBasic	$ 0.67us-gaap_EarningsPerShareBasic
Diluted EPS, Per Share Amount
Diluted earnings per share (in dollars per share)	$ 0.70us-gaap_EarningsPerShareDiluted	$ 0.66us-gaap_EarningsPerShareDiluted
CBIC and Rockbridge
INTANGIBLE ASSETS
Amortization of intangible assets	200,000
Surety segment \| Acquisitions prior to 2011
INTANGIBLE ASSETS
Goodwill and intangibles	26,200,000
Goodwill	25,600,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)	600,000
Surety segment \| Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill	15,100,000
Casualty and Surety segment \| Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill and intangibles	30,500,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)	7,500,000
Definite-lived intangible assets, net of amortization	2,600,000
Casualty segment \| Contractors Bonding and Insurance Company
INTANGIBLE ASSETS
Goodwill	5,300,000
Casualty segment \| Rockbridge
INTANGIBLE ASSETS
Goodwill and intangibles	15,800,000
Goodwill	12,400,000
Definite-lived intangible assets, net of amortization	$ 3,400,000