Debt (Narrative) (Details) (USD $)	0 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended				3 Months Ended		1 Months Ended	3 Months Ended				0 Months Ended	3 Months Ended					0 Months Ended	3 Months Ended
	Mar. 09, 2012	Mar. 31, 2012		Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2011		Mar. 31, 2012 Interest Rate Contract [Member]	Dec. 31, 2011 Interest Rate Contract [Member]	Mar. 31, 2012 Interest Rate Contract One [Member]	Jun. 30, 2010 Interest Rate Contract One [Member]	Oct. 31, 2011 Interest Rate Contract Two [Member]	Mar. 31, 2012 Interest Rate Contract Two [Member]	Jun. 30, 2010 2006 Senior Unsecured Notes [Member]	Mar. 31, 2012 2006 Senior Unsecured Notes [Member]	Dec. 31, 2011 2006 Senior Unsecured Notes [Member]	Feb. 17, 2012 2012 Senior Unsecured Notes [Member]	Mar. 31, 2012 2012 Senior Unsecured Notes [Member]	Mar. 31, 2012 Exchangeable Senior Notes [Member]	Dec. 31, 2011 Exchangeable Senior Notes [Member]	Mar. 31, 2012 Term Loans [Member]	Dec. 31, 2011 Term Loans [Member]	Mar. 09, 2012 2012 Term Loan [Member]	Mar. 31, 2012 2012 Term Loan [Member]	Dec. 31, 2011 Revolving Credit Facilities [Member]	Mar. 09, 2012 Federal Funds Rate [Member]	Mar. 09, 2012 Eurodollar Rate [Member]	Mar. 09, 2012 Maximum [Member] 2012 Term Loan [Member]	Mar. 09, 2012 Minimum [Member] 2012 Term Loan [Member]
Debt Instrument [Line Items]
Mortgage loan assumed, maturity date		Jan. 01, 2018
Unsecured senior notes, value of offering																			$ 200,000,000
Unsecured senior notes, interest rate																			6.375%	6.375%		9.25%	9.25%	6.20%
Unsecured senior note, maturity date																				February 15, 2022
Revolving credit facility																									70,000,000
Revolving credit facilities, balance amount																														400,000,000	330,000,000
Credit facilities, basis spread on variable rate																												0.50%	1.00%
LIBOR plus spread interest rate	2.25%
Initial spread interest rate	1.25%
Term loan facility, maturity period																										Mar. 09, 2016
Net proceeds, after underwriting discount																			196,500,000
Paid off term loan																									100,000,000
Interest rate swap, amount fixed														60,000,000
Total senior notes available		900,225,000	[1]					689,849,000	[1]
Maturity date of interest rate swap												July 2016			October 2016
Interest rate of derivative instrument													5.507%	5.675%	5.675%
Fair value of the interest rate swaps										11,700,000	12,200,000
Other collateral assets		6,700,000						6,300,000
Hedge ineffectiveness and income statement effect in period				0	0	0	0
Debt face amount																	125,000,000	125,000,000		200,000,000	11,000,000	11,000,000					89,600,000
Portion of debt instrument face amount																$ 65,000,000
Percentage of dividends paid from adjusted operating funds		120.00%
Restricted percentage for future dividend payment		115.00%
Restricted percentage for dividend payment for next year		95.00%
Percentage of dividends paid from operation funds		95.00%
Percentage of total unencumbered assets		150.00%						150.00%

[1] Level II: Fair value based on quoted prices for similar or identical instruments in active or inactive markets, respectively, or calculated utilizing model-derived valuations in which significant inputs or value drivers are observable in active markets.