Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		3 Months Ended		6 Months Ended			6 Months Ended
	Jul. 31, 2011	May 31, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	May 10, 2011	Jun. 30, 2011 Series E	Dec. 31, 2010 Series E
Shareholders' Equity
Common shares issued to trustees as part of annual compensation (in shares)		2,000
Stock price per share which is the closing price at NYSE (in dollars per share)							$ 26.57
Number of trustees receiving shares as part of annual compensation		5
Interest rate swap agreements qualifying as cash flow hedges used for hedging mortgage note payable			$ 175,000		$ 175,000
Comprehensive income:
Net income			19,964	9,998	66,576	47,295
Unrealized loss on derivative instrument			(4,119)	(7,941)	(2,074)	(10,815)
Realized gain on sale of investment in available for sale securities					(18)
Foreign currency translation adjustments			11,343		15,705
Increase in share of investees other comprehensive income			40		44
Total comprehensive income			27,228	2,057	80,233	36,480
Cumulative redeemable preferred shares issued (in shares)								11,000,000	0
Proceeds from cumulative redeemable preferred shares								265,804
Liquidation preference (in dollars per share)								$ 25.00
Preferred shares dividend (in dollars per share)								$ 1.8125
Preferred shares, dividend yield (as a percent)								7.25%
Issuance of common shares in public offerings (in shares)	11,500,000
Proceeds from common shares	$ 263,900					$ 239,095