Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 109,984	$ 135,409	$ 164,674
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation	166,444	180,619	155,341
Net amortization of debt discounts, premiums and deferred financing fees	6,943	7,534	6,782
Straight line rental income	(31,474)	(12,200)	(13,368)
Amortization of acquired real estate leases	48,441	34,032	35,174
Other amortization	16,744	16,324	15,206
Loss on asset impairment	10,355	129,264	31,882
Loss (gain) on early extinguishment of debt	35	1,044	(20,686)
Equity in earnings of investees	(11,377)	(8,464)	(6,546)
Gain on issuance of shares by an equity investee	(11,177)	(34,808)
Distributions of earnings from investees	11,238	8,465	4,975
Net gain on sale of properties	(42,752)	(172,104)	(79,133)
Gain on asset acquisition		(20,392)
Change in assets and liabilities:
(Increase) decrease in restricted cash	(2,787)	6,580	(825)
Increase in rents receivable and other assets	(31,362)	(22,580)	(5,650)
Increase in accounts payable and accrued expenses	12,589	3,129	3,034
Increase (decrease) in rent collected in advance	8,479	(1,200)	2,983
Increase in security deposits	1,933	719	5,162
Increase (decrease) in due to related persons	1,076	683	(1,764)
Cash provided by operating activities	263,332	252,054	297,241
CASH FLOWS FROM INVESTING ACTIVITIES:
Real estate acquisitions	(768,060)	(895,549)	(608,621)
Real estate improvements	(100,339)	(77,389)	(57,026)
Investment in direct financing lease, net	(38,635)
Principal payments received from direct financing lease	5,256
Principal payments received from real estate mortgage receivable	8,183	105
Proceeds from investment in marketable pass through certificates		8,000
Investment in marketable pass through certificates			(6,760)
Proceeds from sale of properties, net	264,284	603,800	212,048
Distributions in excess of earnings from investees	5,379	7,654
Investment in Affiliates Insurance Company		(76)	(5,133)
Cash used in investing activities	(623,932)	(353,455)	(465,492)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common shares, net	264,056	430,778
Proceeds from issuance of preferred shares, net	265,391
Redemption of preferred shares		(175,000)
Repurchase and retirement of common shares			(14,486)
Repurchase and retirement of outstanding debt securities			(88,251)
Proceeds from borrowings	1,007,000	1,828,632	1,082,000
Payments on borrowings	(975,030)	(1,618,101)	(632,059)
Deferred financing fees	(7,131)	(13,055)	(17,721)
Distributions to common shareholders	(150,074)	(126,237)	(107,878)
Distributions to preferred shareholders	(44,405)	(49,656)	(50,668)
Purchase of noncontrolling equity interest		(2,500)
Cash provided by financing activities	359,807	274,861	170,937
Effect of exchange rate changes on cash	(484)	2,376
(Decrease) increase in cash and cash equivalents	(1,277)	175,836	2,686
Cash and cash equivalents at beginning of year	194,040	18,204	15,518
Cash and cash equivalents at end of year	192,763	194,040	18,204
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	186,774	173,221	166,771
Taxes paid	966	690	711
NON-CASH INVESTING ACTIVITIES:
Real estate acquisitions	(321,235)		(9,078)
Investment in real estate mortgage receivable		(8,288)
Net assets transferred to Government Properties Income Trust			395,317
Working capital acquired		1,153
NON-CASH FINANCING ACTIVITIES:
Issuance of common shares	1,039	896	628
Assumption of mortgage notes payable	321,235		9,069
Secured credit facility and related deferred financing fees transferred to Government Properties Income Trust			(243,199)
Common distributions declared		$ (26,863)	$ 26,863