Notes Payable (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Debt Instrument [Line Items]
Notes Payable	$ 606,700,000		$ 606,700,000		$ 611,400,000
Interest Expense before Capitalization of Interest	9,700,000	9,800,000	29,300,000	30,100,000
Capitalization of Interest	(200,000)	(200,000)	(700,000)	(1,600,000)
Interest Expense	9,500,000	9,600,000	28,600,000	28,500,000
Interest Paid	500,000	300,000	19,200,000	20,500,000
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest	300,000	300,000	900,000	1,600,000
Interest Paid	400,000	200,000	600,000	1,900,000
Notes Payable under Revolving Credit Agreement [Member] | 2016 Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Length of Agreement			4 years
Line of Credit Facility, Maximum Borrowing Capacity	325,000,000		325,000,000
Line of Credit Facility, Amount Outstanding	0		0		0
Line of Credit Facility, Remaining Borrowing Capacity	325,000,000		325,000,000		325,000,000
Senior Subordinated Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable	357,800,000		357,800,000		357,300,000
Notes Payable, Interest Rate, Stated Percentage	6.00%		6.00%
Interest Expense before Capitalization of Interest	5,500,000	5,500,000	16,600,000	16,600,000
Interest Paid	0	0	10,800,000	10,800,000
Senior Subordinated Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Notes Payable	248,900,000		248,900,000		248,600,000
Notes Payable, Interest Rate, Stated Percentage	6.00%		6.00%
Interest Expense before Capitalization of Interest	3,900,000	3,900,000	11,600,000	11,600,000
Interest Paid	0	0	7,500,000	7,500,000
Mortgage Note Payabe at Amortized Cost [Member]
Debt Instrument [Line Items]
Notes Payable	0		0		5,500,000
Interest Expense before Capitalization of Interest	0	100,000	200,000	300,000
Interest Paid	$ 100,000	$ 100,000	$ 300,000	$ 300,000