Long-Term Debt (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Maturities of outstanding debt
2013	$ 29,631,000
2014	318,115,000
2015	544,470,000
2016	182,559,000
2017	3,037,000
Thereafter	1,124,425,000
Total debt	2,202,237,000	2,380,100,000
Interest costs
Interest costs incurred	160,000,000	178,700,000	177,200,000
Interest costs incurred capitalized	1,400,000	1,700,000	7,000,000
Cash paid for interest	$ 154,100,000	$ 171,800,000	$ 162,400,000