Long-Term Debt (Details 3) (USD $)	12 Months Ended					0 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended	0 Months Ended	12 Months Ended							12 Months Ended			12 Months Ended			1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Aug. 16, 2011 2011 Term Loan Facility, due August 16, 2016	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016	Dec. 31, 2011 2011 Term Loan Facility, due August 16, 2016	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 LIBOR First option	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 LIBOR First option Minimum	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 LIBOR First option Maximum	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 Prime rate Second option	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 Federal funds effective rate Second option	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 Federal funds effective rate Second option Minimum	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 Federal funds effective rate Second option Maximum	Dec. 31, 2012 2011 Term Loan Facility, due August 16, 2016 BBA daily floating rate LIBOR Second option	Aug. 12, 2010 2010 Revolving Credit Facility, due August 12, 2013	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013	Dec. 31, 2011 2010 Revolving Credit Facility, due August 12, 2013	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 Minimum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 Maximum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR First option	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR First option Minimum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR First option Maximum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR Second option	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR Second option Minimum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 LIBOR Second option Maximum	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 Prime rate Second option	Dec. 31, 2012 2010 Revolving Credit Facility, due August 12, 2013 Federal funds effective rate Second option	Dec. 31, 2012 Letters of credit	Mar. 31, 2012 2008 Term Loan Facility, due March 7, 2012	Mar. 31, 2008 2008 Term Loan Facility, due March 7, 2012	Dec. 31, 2012 2008 Term Loan Facility, due March 7, 2012	Dec. 31, 2011 2008 Term Loan Facility, due March 7, 2012	Mar. 07, 2008 2008 Term Loan Facility, due March 7, 2012
Long-term debt obligation
Term of debt instrument						5 years											3 years															4 years
Initial aggregate principal amount						$ 122,500,000																													$ 100,000,000
Outstanding principal paid	69,188,000	136,062,000	46,688,000																												60,000,000
Maximum amount of facility																	70,000,000
Accordion feature																	25,000,000
Maximum amount available for the issuance of letters of credit																	25,000,000
Maximum amount available for the issuance of swingline loans																	5,000,000
Amount of increase to the maximum borrowing capacity that may be requested																		25,000,000
Reference rate							LIBOR	LIBOR	LIBOR			Agent's prime rate	Federal Funds Rate			BBA daily floating rate LIBOR		LIBOR	LIBOR			LIBOR			LIBOR rate			Agent's prime rate	Federal Funds Rate				LIBOR	LIBOR
Additional variable interest rate spread (as a percent)													0.50%			1.00%									1.00%				0.50%
Variable spread based on total debt-to-EBITDA ratio (as a percent)										2.00%	2.75%			1.00%	1.75%								2.25%	3.00%		1.25%	2.00%
Facility fee rate (as a percent)																				0.40%	0.45%
Fronting fee (as a percent)																														0.125%
Letters of credit amount issued	9,900,000																													9,500,000
Remaining amount available for borrowing																		60,500,000
Interest coverage ratio (as a percent)				2.75
Debt-to-EBITDA ratio (as a percent)					3.0
Aggregate percentage limit of consolidated revenue, assets, or EBITDA accounted for by foreign subsidiaries					0.20
Limit of allowed indebtedness of subsidiaries					20,000,000
Future minimum principal payments
2013	12,250,000
2014	15,313,000
2015	18,375,000
2016	64,312,000
2017	0
Total debt	110,250,000	179,438,000					110,250,000	119,438,000										0	0														0	60,000,000
Cash payments for interest	$ 3,200,000	$ 3,200,000	$ 2,800,000