Summary of Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenues	$ 335,451	$ 342,170	$ 350,599	$ 317,703	$ 346,142		$ 314,519	$ 337,726	$ 296,190	$ 1,345,923	$ 1,294,577	$ 1,125,295
Income from operations	22,203	24,668	22,807	10,116	24,065		19,356	12,228	17,365	79,794	73,014	52,293
Net income	$ 17,369	$ 16,473	$ 21,217	$ 12,335	$ 21,227	[1]	$ 10,629	$ 6,345	$ 9,268	$ 67,394	$ 47,469	$ 36,607
Basic income per share (in dollars per share)	$ 0.29	$ 0.28	$ 0.36	$ 0.21	$ 0.36		$ 0.18	$ 0.11	$ 0.16	$ 1.14	$ 0.81	$ 0.64
Diluted income per share (in dollars per share)	$ 0.29	$ 0.28	$ 0.36	$ 0.21	$ 0.36		$ 0.18	$ 0.11	$ 0.16	$ 1.13	$ 0.81	$ 0.63

[1] Includes a reduction in income tax expense resulting from the reinstatement of the Federal research and development tax credit that increased net income by $7.4 million.