Notes Payable (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended				3 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Notes Payable under Revolving Credit Agreement [Member]	Jun. 30, 2011 Notes Payable under Revolving Credit Agreement [Member]	Jun. 30, 2012 Notes Payable under Revolving Credit Agreement [Member]	Jun. 30, 2011 Notes Payable under Revolving Credit Agreement [Member]	Jun. 30, 2012 Notes Payable under Revolving Credit Agreement [Member] 2016 Credit Agreement [Member]	Mar. 07, 2012 Notes Payable under Revolving Credit Agreement [Member] 2016 Credit Agreement [Member]	Mar. 31, 2012 Notes Payable under Revolving Credit Agreement [Member] Former Credit Agreement [Member]	Mar. 07, 2012 Notes Payable under Revolving Credit Agreement [Member] Former Credit Agreement [Member]	Dec. 31, 2011 Notes Payable under Revolving Credit Agreement [Member] Former Credit Agreement [Member]	Jun. 30, 2012 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Jun. 30, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Jun. 30, 2012 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Jun. 30, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Dec. 31, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due May 15, 2017 [Member]	Jun. 30, 2012 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Jun. 30, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Jun. 30, 2012 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Jun. 30, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Dec. 31, 2011 Senior Subordinated Notes [Member] Senior Notes, 6 Percent Due November 30, 2015 [Member]	Jun. 30, 2012 Mortgage Note Payabe at Amortized Cost [Member]	Jun. 30, 2011 Mortgage Note Payabe at Amortized Cost [Member]	Jun. 30, 2012 Mortgage Note Payabe at Amortized Cost [Member]	Jun. 30, 2011 Mortgage Note Payabe at Amortized Cost [Member]	Dec. 31, 2011 Mortgage Note Payabe at Amortized Cost [Member]
Debt Instrument [Line Items]
Notes Payable	$ 611.0		$ 611.0		$ 610.6										$ 357.1		$ 357.1		$ 356.8	$ 248.3		$ 248.3		$ 248.2	$ 5.6		$ 5.6		$ 5.6
Notes Payable, Interest Rate, Stated Percentage															6.00%		6.00%			6.00%		6.00%
Line of Credit Facility, Maximum Borrowing Capacity											325.0		245.0
Line of Credit Facility, Length of Agreement										4 years
Line of Credit Facility, Amount Outstanding										0			0	0
Line of Credit Facility, Remaining Borrowing Capacity										325.0
Write off of Deferred Debt Issuance Cost												0.5
Interest Expense before Capitalization of Interest	9.8	9.9	20.3	19.8		0.3	0.4	1.3	0.9						5.6	5.5	11.1	11.0		3.8	3.9	7.7	7.7		0.1	0.1	0.2	0.2
Capitalization of Interest	(0.6)	(0.6)	(1.4)	(1.1)
Interest Expense	9.2	9.3	18.9	18.7
Interest Paid	$ 18.4	$ 18.7	$ 20.2	$ 19.0		$ 0	$ 0	$ 1.7	$ 0.2						$ 10.8	$ 10.8	$ 10.8	$ 10.8		$ 7.5	$ 7.8	$ 7.5	$ 7.8		$ 0.1	$ 0.1	$ 0.2	$ 0.2