Notes Payable - Interest Expense on Notes Payable (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Interest Costs Incurred	$ 39.8	$ 39.8	$ 34.4
Capitalization of Interest	(1.8)	(2.5)	(1.8)
Total Interest Expense	38.0	37.3	32.6
Notes Payable under Revolving Credit Agreement
Debt Instrument [Line Items]
Interest Costs Incurred	1.9	2.0	2.0
Senior Notes [Member] | Senior Notes, 6% Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	22.1	22.0	22.0
Senior Notes [Member] | Senior Notes, 6% Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	15.4	15.4	1.6
Senior Notes [Member] | Senior Notes, 4.875% Due November 1, 2010 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	0	0	8.4
Mortgage Note Payable [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	$ 0.4	$ 0.4	$ 0.4