Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Components of other comprehensive income (loss) and related tax effects
Amounts reclassified, Net of tax			$ 169	$ 848
Changes in fair value of swaps, net of taxes			(2,555)	819
Other comprehensive income (loss), Gross Total	(4,329)	(3,083)	(3,848)	2,688
Income tax (benefit) expense related to items of other comprehensive income	1,645	1,172	1,462	(1,021)
Other comprehensive income (loss), Total, Net of tax	(2,684)	(1,911)	(2,386)	1,667
Interest Rate Swap [Member]
Components of other comprehensive income (loss) and related tax effects
Amounts reclassified, gross	1,366	1,589	2,506	3,455
Amounts reclassified, Tax effect	(519)	(604)	(952)	(1,313)
Amounts reclassified, Net of tax	847	985	1,554	2,142
Changes in fair value, gross	(3,791)	(2,670)	(4,793)	(2,603)
Changes in fair value, Tax effect	1,441	1,015	1,821	989
Changes in fair value of swaps, net of taxes	(2,350)	(1,655)	(2,972)	(1,614)
Fuel Hedges [Member]
Components of other comprehensive income (loss) and related tax effects
Amounts reclassified, gross	(1,105)	(1,278)	(2,234)	(2,088)
Amounts reclassified, Tax effect	420	486	849	794
Amounts reclassified, Net of tax	(685)	(792)	(1,385)	(1,294)
Changes in fair value, gross	(799)	(724)	673	3,924
Changes in fair value, Tax effect	303	275	(256)	(1,491)
Changes in fair value of swaps, net of taxes	$ (496)	$ (449)	$ 417	$ 2,433