Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Contract revenue	$ 231,979	$ 237,864	$ 239,649	$ 227,641	$ 213,947	$ 218,691	$ 213,395	$ 194,742	$ 937,133	$ 840,775	$ 764,734
Operating income	14,426	16,875	18,065	16,254	13,911	16,099	15,535	13,437	65,620	58,982	46,920
Net income	$ 9,219	$ 9,573	$ 10,346	$ 8,937	$ 8,842	$ 9,334	$ 8,963	$ 7,726	$ 38,075	$ 34,865	$ 27,171
Earnings per Share:
Basic	$ 0.47	$ 0.49	$ 0.52	$ 0.45	$ 0.45	$ 0.47	$ 0.46	$ 0.39	$ 1.94	$ 1.77	$ 1.40
Diluted	$ 0.47	$ 0.48	$ 0.52	$ 0.45	$ 0.44	$ 0.47	$ 0.45	$ 0.39	$ 1.91	$ 1.75	$ 1.38
Weighted-average Common Shares Outstanding:
Basic	19,501	19,610	19,774	19,769	19,738	19,728	19,688	19,580	19,663	19,684	19,375
Diluted	19,690	19,770	19,971	20,082	19,956	19,860	19,847	19,780	19,957	19,928	19,626