4. Debt Unsecured Credit Facility (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Extension Option On Debt Maturity Years	1
Line of credit	$ 0	$ 18,000,000
Letters of Credit Outstanding, Amount	0
Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Line Of Credit Capacity Prior To Second Amendment	225,000,000
Line of Credit Facility, Maximum Borrowing Capacity	400,000,000
Debt Instrument, Maturity Date	Mar. 21, 2016
Basis Points Extension Fee On Total Commitment	25
Facility amount available for Letters of Credit	35,000,000
Line Of Credit Commitment If Increased	$ 600,000,000