Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Comprehensive Income [Abstract]
Net income	$ 23,176	$ 195,867	$ 146,484
Change in value of derivative instruments used as cash flow hedges, net of tax of $12,094, $18,412, and $13,254	18,635	29,384	21,392
Reclassification - derivative settlements, net of tax of ($20,171). ($1,146), and ($19,987)	(31,682)	(1,819)	(32,268)
Ineffective portion of derivatives, net of tax of $1,008, ($1,061), and ($267)	1,608	(1,688)	(433)
Total comprehensive income	$ 11,737	$ 221,744	$ 135,175