Unsecured Notes Payable (Details) (USD $)	0 Months Ended	12 Months Ended		0 Months Ended					0 Months Ended						0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended
	Dec. 19, 2011 years	Dec. 31, 2011	Dec. 31, 2010	Dec. 09, 2011 Unsecured Term Loans [Member]	Dec. 09, 2011 Unsecured Term Loans [Member]	Dec. 09, 2011 Unsecured Term Loans [Member] years	Oct. 28, 2011 Unsecured Term Loans [Member]	Dec. 31, 2011 Unsecured Term Loans [Member]	Dec. 21, 2011 Senior Notes [Member]	Dec. 19, 2011 Senior Notes [Member] years	Nov. 01, 2011 Senior Notes [Member]	Dec. 31, 2011 Senior Notes [Member]	Dec. 31, 2008 Senior Notes [Member]	Oct. 31, 2006 Senior Notes [Member]	Dec. 19, 2011 Series A [Member]	Dec. 31, 2011 Series A [Member]	Dec. 19, 2011 Series B [Member]	Dec. 31, 2011 Series B [Member]
Debt Instrument [Line Items]
Term loan total limit							$ 140,000,000
Unsecured term loan agreement			140,000,000	250,000,000	250,000,000	250,000,000						0	140,000,000
Unsecured term loan borrowed facility		135,000,000			135,000,000				5,000,000		135,000,000
Proceeds from term loan				248,215,000
Proceeds from unsecured term loan		400,000,000			250,000,000
Unsecured debt fixed interest percentage							2.41%							4.125%	4.46%		5.00%
Percentage of interest above the one-month LIBOR				1.30%	1.30%	1.30%	2.10%
One-month LIBOR rate								0.31%
Line of credit facility effective rate		1.61%						1.61%
Unsecured senior notes issuance date		December 19, 2011
Unsecured senior notes issued		400,000,000								150,000,000					90,000,000		60,000,000
Unsecured senior notes, maturity date																Dec. 19, 2018		Dec. 19, 2021
Unsecured notes, maturity term (in years)	7					5				10
Proceeds from issuance of unsecured senior notes															$ 89,449,000		$ 59,629,000