Investments in Unconsolidated Joint Ventures (Summary Financial Data) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Real estate assets	$ 1,473,229	$ 1,694,165
Accumulated depreciation	(300,503)	(285,535)
Real estate assets, net	1,172,726	1,408,630
Development in progress	677,918	398,892
Land held for development	73,826	77,556
Other assets	200,663	209,271
Total assets	2,125,133	2,094,349
Liabilities and Equity
Debt	846,328	1,061,218
Other liabilities	166,697	136,847
Equity	1,112,108	896,284
Total liabilities and equity	2,125,133	2,094,349
Company's net investment in unconsolidated joint ventures	245,078	218,454
Condensed Statements of Operations
Total revenue	178,876	211,327	$ 203,006
Operating expense	67,427	82,565	75,752
Operating income (loss)	111,449	128,762	127,254
Interest	(47,044)	(59,509)	(61,326)
Depreciation and amortization	(44,709)	(54,766)	(60,866)
Other income (expense)	8,474	55,960	9,404
Gain (loss) on sale/impairment	45,000	(56,032)	(172,504)
Net income (loss)	73,170	14,415	(158,038)
Company's equity in earnings (loss) of unconsolidated joint ventures	21,970	3,149	10,314
Difference between carrying amount and underlying equity	1,400	4,500
Liberty Venture I, LP [Member]
Assets
Real estate assets	226,313	168,690
Accumulated depreciation	(42,779)	(37,473)
Real estate assets, net	183,534	131,217
Development in progress	6,886	34,878
Land held for development	0	4,471
Other assets	23,494	14,860
Total assets	213,914	185,426
Liabilities and Equity
Debt	154,386	121,793
Other liabilities	6,673	10,571
Equity	52,855	53,062
Total liabilities and equity	213,914	185,426
Company's net investment in unconsolidated joint ventures	11,175	11,563
Condensed Statements of Operations
Total revenue	25,149	23,708	19,277
Operating expense	6,835	7,977	6,398
Operating income (loss)	18,314	15,731	12,879
Interest	(4,182)	(5,415)	(5,078)
Depreciation and amortization	(6,375)	(6,399)	(5,757)
Other income (expense)	50	68	(17)
Gain (loss) on sale/impairment	0	760	0
Net income (loss)	7,807	4,745	2,027
Company's equity in earnings (loss) of unconsolidated joint ventures	2,332	1,540	788
Kings Hill Unit Trust [Member]
Assets
Real estate assets	137,960	164,603
Accumulated depreciation	(26,872)	(28,513)
Real estate assets, net	111,088	136,090
Development in progress	0	0
Land held for development	0	0
Other assets	9,603	8,967
Total assets	120,691	145,057
Liabilities and Equity
Debt	83,846	99,603
Other liabilities	24,513	27,408
Equity	12,332	18,046
Total liabilities and equity	120,691	145,057
Company's net investment in unconsolidated joint ventures	5,349	6,789
Condensed Statements of Operations
Total revenue	11,926	12,602	13,221
Operating expense	4,854	5,261	5,387
Operating income (loss)	7,072	7,341	7,834
Interest	(2,554)	(6,116)	(6,110)
Depreciation and amortization	(3,557)	(3,954)	(4,135)
Other income (expense)	(77)	45,604	208
Gain (loss) on sale/impairment	0	0	0
Net income (loss)	884	42,875	(2,203)
Company's equity in earnings (loss) of unconsolidated joint ventures	275	(425)	(288)
Liberty Illinois, LP [Member]
Assets
Real estate assets	251,765	263,552
Accumulated depreciation	(51,901)	(54,308)
Real estate assets, net	199,864	209,244
Development in progress	0	22,998
Land held for development	31,567	28,590
Other assets	19,036	20,668
Total assets	250,467	281,500
Liabilities and Equity
Debt	137,294	155,601
Other liabilities	8,433	8,594
Equity	104,740	117,305
Total liabilities and equity	250,467	281,500
Company's net investment in unconsolidated joint ventures	15,339	18,221
Condensed Statements of Operations
Total revenue	26,599	26,085	25,181
Operating expense	9,961	9,303	8,965
Operating income (loss)	16,638	16,782	16,216
Interest	(7,243)	(8,441)	(7,932)
Depreciation and amortization	(7,589)	(7,406)	(7,380)
Other income (expense)	64	29	39
Gain (loss) on sale/impairment	4,068	0	187
Net income (loss)	5,938	964	1,130
Company's equity in earnings (loss) of unconsolidated joint ventures	2,065	807	824
Liberty Washington, LP [Member]
Assets
Real estate assets	289,627	534,132
Accumulated depreciation	(44,235)	(43,224)
Real estate assets, net	245,392	490,908
Development in progress	0	0
Land held for development	0	2,000
Other assets	18,749	52,429
Total assets	264,141	545,337
Liabilities and Equity
Debt	103,667	314,080
Other liabilities	4,385	10,634
Equity	156,089	220,623
Total liabilities and equity	264,141	545,337
Company's net investment in unconsolidated joint ventures	38,863	55,552
Condensed Statements of Operations
Total revenue	39,727	71,101	72,824
Operating expense	15,935	27,384	28,349
Operating income (loss)	23,792	43,717	44,475
Interest	(10,866)	(17,353)	(18,612)
Depreciation and amortization	(10,750)	(20,725)	(27,112)
Other income (expense)	257	531	375
Gain (loss) on sale/impairment	40,943	(56,792)	(172,691)
Net income (loss)	43,376	(50,622)	(173,565)
Company's equity in earnings (loss) of unconsolidated joint ventures	10,857	(7,314)	2,251
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Assets
Real estate assets	495,842	495,248
Accumulated depreciation	(125,014)	(111,067)
Real estate assets, net	370,828	384,181
Development in progress	0	0
Land held for development	0	0
Other assets	48,074	47,441
Total assets	418,902	431,622
Liabilities and Equity
Debt	309,475	313,402
Other liabilities	9,964	9,618
Equity	99,463	108,602
Total liabilities and equity	418,902	431,622
Company's net investment in unconsolidated joint ventures	18,688	20,465
Condensed Statements of Operations
Total revenue	65,587	68,444	63,580
Operating expense	26,761	29,692	23,557
Operating income (loss)	38,826	38,752	40,023
Interest	(19,737)	(19,936)	(20,179)
Depreciation and amortization	(14,513)	(14,442)	(14,591)
Other income (expense)	(213)	(227)	(278)
Gain (loss) on sale/impairment	0	0	0
Net income (loss)	4,363	4,147	4,975
Company's equity in earnings (loss) of unconsolidated joint ventures	1,594	2,053	1,633
Liberty Property 18th & Arch [Member]
Assets
Real estate assets	0	0
Accumulated depreciation	0	0
Real estate assets, net	0	0
Development in progress	671,032	323,615
Land held for development	0	0
Other assets	52,053	15,594
Total assets	723,085	339,209
Liabilities and Equity
Debt	0	0
Other liabilities	95,574	40,958
Equity	627,511	298,251
Total liabilities and equity	723,085	339,209
Company's net investment in unconsolidated joint ventures	132,952	62,677
Condensed Statements of Operations
Total revenue	0	0	0
Operating expense	280	221	199
Operating income (loss)	(280)	(221)	(199)
Interest	0	0	0
Depreciation and amortization	0	0	0
Other income (expense)	1,270	30	(13)
Gain (loss) on sale/impairment	0	0	0
Net income (loss)	990	(191)	(212)
Company's equity in earnings (loss) of unconsolidated joint ventures	213	(29)	(33)
Other Joint Ventures [Member]
Assets
Real estate assets	71,722	67,940
Accumulated depreciation	(9,702)	(10,950)
Real estate assets, net	62,020	56,990
Development in progress	0	17,401
Land held for development	42,259	42,495
Other assets	29,654	49,312
Total assets	133,933	166,198
Liabilities and Equity
Debt	57,660	56,739
Other liabilities	17,155	29,064
Equity	59,118	80,395
Total liabilities and equity	133,933	166,198
Company's net investment in unconsolidated joint ventures	22,712	43,187
Condensed Statements of Operations
Total revenue	9,888	9,387	8,923
Operating expense	2,801	2,727	2,897
Operating income (loss)	7,087	6,660	6,026
Interest	(2,462)	(2,248)	(3,415)
Depreciation and amortization	(1,925)	(1,840)	(1,891)
Other income (expense)	7,123	9,925	9,090
Gain (loss) on sale/impairment	(11)	0	0
Net income (loss)	9,812	12,497	9,810
Company's equity in earnings (loss) of unconsolidated joint ventures	4,634	6,517	5,139
Cambridge Medipark Ltd [Member] | Leaseholds and Leasehold Improvements [Member]
Condensed Statements of Operations
Gain (loss) on sale/impairment	$ 7,100	$ 9,900	$ 9,100