Other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (801,554)	$ (874,338)
Treasury Rate Lock [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	96	180	263
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	0	0	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		0
Other Comprehensive Income (Loss), Net of Tax	(96)	(84)	(83)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	151,780	46,087	71,378	124,510
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	200,699	140,985	186,824
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	(95,006)	(166,276)	239,956
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		0
Other Comprehensive Income (Loss), Net of Tax	0	0	0
Adjustment For Actuarially Determined Liabilities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(953,334)	(920,521)	(699,293)	(564,744)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	0	0	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		0
Other Comprehensive Income (Loss), Net of Tax	(32,813)	(221,228)	(134,549)
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	0	(12,769)	(21,929)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	0	(12,500)	(31,162)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	0	7,248	40,024
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		18,026
Other Comprehensive Income (Loss), Net of Tax	0	(5)	298
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(801,554)	(874,338)	(640,504)	(461,900)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	200,699	128,540	154,700
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	(95,006)	(159,083)	198,970
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		18,026
Other Comprehensive Income (Loss), Net of Tax	$ (32,909)	$ (221,317)	$ (134,334)