Comprehensive Income (Details) (USD $) In Thousands, 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
Components of comprehensive income [Abstract]
Net income	$ (10,115)	$ 24,169	$ 58,577	$ (638)	$ (11,805)	$ 22,784	$ 52,770	$ (6,111)	$ 71,993	$ 57,638	$ 19,202
Foreign currency translation adjustments									(609)	(2,170)	2,434
Net change - Unrealized gains (losses) on interest rate swaps									1,777	996	2,178
Comprehensive income									73,161	56,464	23,814
Components of and changes in accumulated other comprehensive income [Abstract]
Foreign currency translation				3,085				5,255	3,085	5,255
Net change - Foreign currency translation									(609)	(2,170)
Foreign currency translation	2,476				3,085				2,476	3,085	5,255
Unrealized gains (losses) on interest rate swaps				(2,033)				(3,029)	(2,033)	(3,029)
Net change - Unrealized gains (losses) on interest rate swaps									1,777	996	2,178
Unrealized gains (losses) on interest rate swaps	(256)				(2,033)				(256)	(2,033)	(3,029)
Total accumulated other comprehensive income (loss)	2,220				1,052				2,220	1,052	2,226
Net change - Total accumulated other comprehensive income (loss)									1,168	(1,174)
Total accumulated other comprehensive income (loss)	$ 2,220				$ 1,052				$ 2,220	$ 1,052	$ 2,226