Notes Payable (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Debt Instrument [Line Items]
Notes Payable	$ 611.6		$ 611.4
Interest Expense before Capitalization of Interest	9.8	10.5
Capitalization of Interest	(0.3)	(0.8)
Interest Expense	9.5	9.7
Interest Paid	0.1	1.8
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Interest Expense before Capitalization of Interest	0.3	1.0
Interest Paid	0	1.7
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, Amount Outstanding	0		0
Line of Credit Facility, Remaining Borrowing Capacity	325.0		325.0
Senior Subordinated Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Notes Payable	357.4		357.3
Notes Payable, Interest Rate, Stated Percentage	6.00%
Interest Expense before Capitalization of Interest	5.5	5.5
Senior Subordinated Notes [Member] | Senior Notes, 6 Percent Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Notes Payable	248.7		248.6
Notes Payable, Interest Rate, Stated Percentage	6.00%
Interest Expense before Capitalization of Interest	3.9	3.9
Mortgage Note Payabe at Amortized Cost [Member]
Debt Instrument [Line Items]
Notes Payable	5.5		5.5
Interest Expense before Capitalization of Interest	0.1	0.1
Interest Paid	$ 0.1	$ 0.1