Notes Payable - Debt Outstanding (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Notes Payable under Revolving Credit Agreement Former Credit Agreement [Member]	Mar. 07, 2012 Notes Payable under Revolving Credit Agreement Former Credit Agreement [Member]	Dec. 31, 2011 Notes Payable under Revolving Credit Agreement Former Credit Agreement [Member]	Dec. 31, 2013 Senior Notes [Member] Senior Notes, 6% Due May 15, 2017 [Member]	Dec. 31, 2012 Senior Notes [Member] Senior Notes, 6% Due May 15, 2017 [Member]	Dec. 31, 2013 Senior Notes [Member] Senior Notes, 6% Due November 30, 2015 [Member]	Dec. 31, 2012 Senior Notes [Member] Senior Notes, 6% Due November 30, 2015 [Member]	Dec. 31, 2013 Mortgage Note Payable [Member]	Dec. 31, 2012 Mortgage Note Payable [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 245,000,000.0
Notes Payable	606,900,000	611,400,000				357,900,000	357,300,000	249,000,000	248,600,000	0	5,500,000
Line of Credit Facility, Amount Outstanding				$ 0	$ 0