Accumulated Other Comprehensive Loss 1 (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Comprehensive Income Loss [Abstract]
Net earnings	$ 76,957	$ 58,032	$ 59,773	$ 60,662	$ 255,424	$ 224,548	$ 179,580
Changes in other comprehensive income (loss):
Change in fair value of interest rate hedges, net of tax of $(27,589), $(5,237) and $(3,051), respectively					(46,470)	(7,790)	(4,538)
Reclassification of unrealized loss (gain) on interest rate hedges into earnings, net of tax of $9,012, $9,019 and $8,222, respectively					13,443	13,463	12,350
Change in fair value of commodity hedges, net of tax of $2,598, $14,093 and $3,773, respectively					4,610	25,012	6,696
Reclassification of unrealized (gain) loss on commodity hedges into earnings, net of tax of $(8,536), $(6,787) and $(16,231), respectively					(15,149)	(12,046)	(28,804)
Actuarial gain (loss) relating to pension and other postretirement benefits, net of tax of $(24,240), $(4,472) and $6,535, respectively					(38,786)	(5,319)	8,185
Prior service cost relating to pension and other postretirement benefit plan amendments, net of tax of $0, $0 and $(151), respectively					0	0	(186)
Reclassification of net actuarial loss and prior service credit relating to pension and other postretirement benefits into earnings net of tax of $2,234, $2,205 and $2,814, respectively					3,174	2,886	4,035
Change in other comprehensive income (loss) from equity investments net of tax of $88, $88 and $(1,744), respectively					143	142	(2,820)
Total other comprehensive income (loss)					(79,035)	16,348	(5,082)
Total comprehensive income					$ 176,389		$ 174,498