Debt (Revolving Credit Facilities) (Details) (USD $)	12 Months Ended						1 Months Ended	12 Months Ended			12 Months Ended					12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 Revolving Credit Facilities [Member]	Dec. 31, 2011 Revolving Credit Facilities [Member]	Dec. 31, 2009 Revolving Credit Facilities [Member]	Jun. 30, 2007 Collateralized Revolving Bank Credit Facility [Member]	Dec. 31, 2011 Collateralized Revolving Bank Credit Facility [Member]	Dec. 31, 2010 Collateralized Revolving Bank Credit Facility [Member]	Dec. 31, 2009 Collateralized Revolving Bank Credit Facility [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 LIBOR [Member]	Dec. 31, 2009 LIBOR [Member] Revolving Credit Facilities [Member]	Jun. 30, 2007 LIBOR [Member] Collateralized Revolving Bank Credit Facility [Member]	Dec. 31, 2011 Federal Funds Rate [Member] Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 Minimum [Member] Revolving Credit Facilities [Member]	Dec. 31, 2009 Minimum [Member] Revolving Credit Facilities [Member]	Dec. 31, 2011 Minimum [Member] Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 Minimum [Member] LIBOR [Member]	Dec. 31, 2011 Maximum [Member] Revolving Credit Facilities [Member]	Dec. 31, 2009 Maximum [Member] Revolving Credit Facilities [Member]	Dec. 31, 2011 Maximum [Member] Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 Maximum [Member] LIBOR [Member]
Debt Instrument [Line Items]
Credit facility, maturity date											October 2015
Credit facilities, basis spread of interest rate						1.99%		1.80%	1.77%		1.60%	2.60%	1.75%	1.50%	0.50%			1.60%	2.60%			2.40%	3.40%
Credit facilities, decrease								$ 800,000
Credit facility paid	246,262,000	216,765,000	1,232,000	220,000,000
Credit facilities, amount outstanding				0	50,000,000	96,000,000	42,000,000	39,600,000	0		330,000,000
Credit facilities, repayment term							five years
Credit facilities, percentage of commitment fee on unused capacity																0.375%	0.20%			0.50%	0.35%
Credit facilities, remaining borrowing capacity								0
Credit facilities, weighted average interest rate				2.10%	3.20%	2.21%		1.63%	1.74%	1.86%
Collateralized real estate property								55,100,000	56,500,000
Revolving facility agreement, availability					$ 280,000,000