Revolving Credit Facility (Details) (USD $)			3 Months Ended							3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Revolving Credit Facility multiplier	Mar. 31, 2013 Revolving Credit Facility Minimum	Mar. 31, 2013 Revolving Credit Facility Maximum	Mar. 31, 2013 Revolving Credit Facility Prime rate	Mar. 31, 2012 Revolving Credit Facility Prime rate	Mar. 31, 2013 Revolving Credit Facility Prime rate Minimum	Mar. 31, 2013 Revolving Credit Facility Prime rate Maximum	Mar. 31, 2013 Revolving Credit Facility LIBOR	Mar. 31, 2012 Revolving Credit Facility LIBOR	Mar. 31, 2013 Revolving Credit Facility LIBOR Minimum	Mar. 31, 2013 Revolving Credit Facility LIBOR Maximum
Revolving credit facility
Revolving credit limit			$ 125,000,000
Potential future expansion of the Facility under accordion feature			175,000,000
Variable rate basis						Bank's prime rate plus a margin				LIBOR
Interest rate margin per debt instrument (as a percent)								1.25%	2.25%			2.25%	3.25%
Actual interest rate margin (as a percent)						1.25%				2.25%
Weighted average interest rate (as a percent)						4.50%	4.50%			2.54%	2.76%
Quarterly commitment fee on the average daily unused facility balance (as a percent)				0.30%	0.50%
EBITDA multiplier			3.0
Percentage of acquired EBITDA which can be included in covenants calculation					50.00%
Irrevocable letters of credit outstanding			6,300,000
Borrowing capacity permitted to be used per formula			89,100,000
Amount outstanding			0
Remaining borrowing capacity available for use			81,700,000
Minimum net worth required to be maintained			158,100,000
Company's net worth	$ 207,849,000	$ 204,315,000