Quarterly Results of Operations (Unaudited) (Details) (USD $) Share data 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
Quarterly Results of Operations (Unaudited) [Abstract]
Revenues	$ 671,971,000		$ 615,728,000	$ 583,299,000	$ 586,920,000		$ 578,224,000		$ 575,789,000	$ 565,518,000	$ 561,004,000	$ 2,457,918,000	[1]	$ 2,280,535,000	[1]	$ 2,100,274,000	[1]
Income from operations	19,812,000	[2]	29,505,000	28,863,000	12,000,000	[2]	17,772,000	[2]	8,640,000	23,744,000	15,838,000	90,180,000		65,994,000		29,780,000
Net loss	(14,875,000)		(7,036,000)	(33,959,000)	(12,305,000)		(8,136,000)		(16,913,000)	(9,557,000)	(14,295,000)	(65,835,000)		(80,333,000)		(99,181,000)
Weighted average basic and diluted loss per share (in dollars per share)	$ (0.12)		$ (0.06)	$ (0.28)	$ (0.10)		$ (0.07)		$ (0.14)	$ (0.08)	$ (0.12)	$ (0.56)		$ (0.41)		$ (0.60)
Weighted average shares used in computing basic and diluted loss per share (in shares)	120,951		121,616	121,280	120,792		120,580		120,404	119,721	119,315
Non-cash impairment charges included in quarterly results	$ 2,000,000				$ 14,800,000		$ 13,100,000

[1] All revenue is earned from external third parties in the United States.

[2] Fourth quarter 2011 and 2010 results include non-cash impairment charges of $2.0 million and $13.1 million, respectively. First quarter 2011 results include non-cash impairment charges of $14.8 million.