Consolidated Statements Of Comprehensive Income (USD $) In Thousands, 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
Statement of Other Comprehensive Income [Abstract]
Net income	$ (17,382)	$ 66,201	$ (5,264)	$ 73,356	$ 79,469	$ (3,782)	$ 57,251	$ 58,226	$ 116,911	$ 191,164	$ 152,198
Other comprehensive income (loss), before tax:
Gains (losses) on hedging instrument									0	1,139	(220)
Other comprehensive income (loss), before tax									0	1,139	(220)
Income tax expense (benefit) related to gains (losses) on hedging instrument									0	399	(77)
Other comprehensive income (loss), net of tax:									0	740	(143)
Comprehensive income									$ 116,911	$ 191,904	$ 152,055