Fair Value Measurements (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Fair Value Measurements (Details Textual)
Number of notes receivable	2
Notes receivables, carrying value	$ 130,400,000
Notes receivable, fair value	162,000,000
Total Debt Outstanding	1,165,346,000	1,159,215,000
Accrued interest on line of credit facility	LIBOR plus 2.50% or a base rate plus 0.50%
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	13,422,000
Total assets measured at fair value	13,422,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	220,000
Variable to fixed interest rate swaps	1,529,000
Total liabilities measured at fair value	1,749,000
Fair Value, Inputs, Level 1 [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	13,422,000
Total assets measured at fair value	13,422,000
Fair Value, Inputs, Level 2 [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	220,000
Variable to fixed interest rate swaps	1,529,000
Total liabilities measured at fair value	1,749,000
$1.0 Billion Credit Facility [Member]
Fair Value Measurements (Details Textual)
Total credit facility	1,000,000,000
Total Debt Outstanding	700,000,000
Fair value of borrowings outstanding based upon the present value of cash flows discounted at current market interest rates	682,000,000
Convertible Notes Payable [Member]
Fair Value Measurements (Details Textual)
Total Debt Outstanding	312,767,000
Face amount of outstanding debt	360,000,000
Accrued interest on convertible notes	3.75%
Fair value of the Convertible Notes	334,000,000
Senior Notes, 6.75%, Due 2014 [Member]
Fair Value Measurements (Details Textual)
Total Debt Outstanding	152,180,000	152,180,000
Accrued interest on convertible notes	6.75%
Fair value of senior notes	$ 152,900,000