Debt (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009
Schedule on basis of which debt matures
2012	$ 66,063,000
2013	163,003,000
2014	820,780,000
2015	806,104,000
2016	278,642,000
Thereafter	303,879,000
Total	2,438,471,000
Debt maturing in 2012 that may be extended for one year	16,800,000
Extension option period for portion of debt maturing in 2012 (in years)	1
Debt maturing in 2013 that may be extended for one year	17,900,000
Extension option period for portion of debt maturing in 2013 (in years)	1
Debt maturing in 2014 that may be extended for one year	662,000,000
Extension option period for portion of debt maturing in 2014 (in years)	1
Debt maturing in 2015 that may be extended for one year	405,600,000
Extension option period for portion of debt maturing in 2015 (in years)	1
Net discounts	12,200,000
Capitalized interest costs	$ 17,400,000	$ 16,500,000	$ 15,500,000