Comprehensive Income (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Net income	$ 63,325,000	$ 55,543,000	$ 157,593,000	$ 172,253,000
Current period change in fair value of derivative instruments, net of tax	49,449,000	42,917,000	(47,425,000)	98,385,000
Reclassification adjustment for derivative instruments, net of tax	4,037,000	(35,241,000)	354,000	(59,066,000)
Pension and postretirement plans, net of tax	683,000	630,000	1,365,000	1,258,000
Comprehensive income	117,494,000	63,849,000	111,887,000	212,830,000
Current period change in fair value of derivative instruments, tax	30,300,000	26,300,000	(29,100,000)	60,300,000
Reclassification adjustement for derivative instruments, tax	2,500,000	(21,600,000)	200,000	(36,200,000)
Pension and postretirement plans, tax	400,000	300,000	700,000	700,000
Accumulated other comprehensive loss, net of tax
Unrealized loss on hedges, net	(90,738,000)		(90,738,000)		(43,667,000)
Pension and postretirement plans	(29,365,000)		(29,365,000)		(30,730,000)
Accumulated other comprehensive loss	(120,103,000)		(120,103,000)		(74,397,000)
Unrealized loss on hedges, tax	(55,600,000)		(55,600,000)		(26,800,000)
Pension and postretirement plans, tax	$ (15,800,000)		$ (15,800,000)		$ (16,500,000)