Comprehensive Income (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Net income	$ 87,599,000	$ 38,304,000	$ 245,192,000	$ 210,557,000
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Current period change in fair value of derivative instruments, net of tax	172,430,000	(7,244,000)	125,005,000	91,141,000
Reclassification adjustment for derivative instruments, net of tax	(10,440,000)	(33,849,000)	(10,085,000)	(92,914,000)
Pension and postretirement plans, net of tax	(10,438,000)	(6,678,000)	(9,073,000)	(5,420,000)
Comprehensive income	239,151,000	(9,467,000)	351,039,000	203,364,000
Other Comprehensive Income (Loss), Tax [Abstract]
Current period change in fair value of derivative instruments, tax	105,700,000	(4,400,000)	76,600,000	55,900,000
Reclassification adjustement for derivative instruments, tax	(6,400,000)	(20,700,000)	(6,200,000)	(56,900,000)
Pension and postretirement plans, tax	(5,600,000)	(3,600,000)	(4,900,000)	(2,900,000)
Accumulated other comprehensive loss, net of tax
Unrealized gain (loss) on hedges, net	71,253,000		71,253,000		(43,667,000)
Pension and postretirement plans	(39,803,000)		(39,803,000)		(30,730,000)
Accumulated other comprehensive loss	31,450,000		31,450,000		(74,397,000)
Unrealized loss on hedges, tax	43,700,000		43,700,000		(26,800,000)
Pension and postretirement plans, tax	$ (21,400,000)		$ (21,400,000)		$ (16,500,000)