Investments in Unconsolidated Joint Ventures (Summary Financial Data) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Real estate assets		$ 2,056,711	$ 1,473,229
Accumulated depreciation		(339,215)	(300,503)
Real estate assets, net		1,717,496	1,172,726
Development in progress		308,104	677,918
Land held for development		74,575	73,826
Other assets		238,791	200,663
Total assets		2,338,966	2,125,133
Liabilities and Equity
Debt		869,141	846,328
Other liabilities		165,565	166,697
Equity		1,304,260	1,112,108
Total liabilities and equity		2,338,966	2,125,133
Company's net investment in unconsolidated joint ventures	[1]	288,456	245,078
Condensed Statements of Operations
Total revenue		186,239	178,876	$ 211,327
Operating expense		63,249	67,427	82,565
Operating income (loss)		122,990	111,449	128,762
Interest		(40,295)	(47,044)	(59,509)
Depreciation and amortization		(44,366)	(44,709)	(54,766)
Other income (expense)		14,437	8,474	55,960
Gain (loss) on sale/impairment		(15,880)	45,000	(56,032)
Net income (loss)		36,886	73,170	14,415
Company's equity in earnings (loss) of unconsolidated joint ventures		17,155	21,970	3,149
Difference between carrying amount and underlying equity		(3,000)	1,400
Liberty Venture I, LP [Member]
Assets
Real estate assets		229,600	226,313
Accumulated depreciation		(48,629)	(42,779)
Real estate assets, net		180,971	183,534
Development in progress		18,337	6,886
Land held for development		0	0
Other assets		20,722	23,494
Total assets		220,030	213,914
Liabilities and Equity
Debt		166,295	154,386
Other liabilities		4,117	6,673
Equity		49,618	52,855
Total liabilities and equity		220,030	213,914
Company's net investment in unconsolidated joint ventures	[1]	10,425	11,175
Condensed Statements of Operations
Total revenue		31,079	25,149	23,708
Operating expense		8,101	6,835	7,977
Operating income (loss)		22,978	18,314	15,731
Interest		(5,801)	(4,182)	(5,415)
Depreciation and amortization		(7,598)	(6,375)	(6,399)
Other income (expense)		(100)	50	68
Gain (loss) on sale/impairment		0	0	760
Net income (loss)		9,479	7,807	4,745
Company's equity in earnings (loss) of unconsolidated joint ventures		2,914	2,332	1,540
Kings Hill Unit Trust [Member]
Assets
Real estate assets		152,998	137,960
Accumulated depreciation		(30,028)	(26,872)
Real estate assets, net		122,970	111,088
Development in progress		0	0
Land held for development		0	0
Other assets		10,957	9,603
Total assets		133,927	120,691
Liabilities and Equity
Debt		92,131	83,846
Other liabilities		28,056	24,513
Equity		13,740	12,332
Total liabilities and equity		133,927	120,691
Company's net investment in unconsolidated joint ventures	[1]	6,100	5,349
Condensed Statements of Operations
Total revenue		12,976	11,926	12,602
Operating expense		4,756	4,854	5,261
Operating income (loss)		8,220	7,072	7,341
Interest		(2,329)	(2,554)	(6,116)
Depreciation and amortization		(3,238)	(3,557)	(3,954)
Other income (expense)		(4)	(77)	45,604
Gain (loss) on sale/impairment		0	0	0
Net income (loss)		2,649	884	42,875
Company's equity in earnings (loss) of unconsolidated joint ventures		660	275	(425)
Liberty Illinois, LP [Member]
Assets
Real estate assets		252,593	251,765
Accumulated depreciation		(57,444)	(51,901)
Real estate assets, net		195,149	199,864
Development in progress		0	0
Land held for development		33,500	31,567
Other assets		16,337	19,036
Total assets		244,986	250,467
Liabilities and Equity
Debt		138,869	137,294
Other liabilities		7,548	8,433
Equity		98,569	104,740
Total liabilities and equity		244,986	250,467
Company's net investment in unconsolidated joint ventures	[1]	14,980	15,339
Condensed Statements of Operations
Total revenue		23,711	26,599	26,085
Operating expense		7,989	9,961	9,303
Operating income (loss)		15,722	16,638	16,782
Interest		(5,882)	(7,243)	(8,441)
Depreciation and amortization		(6,918)	(7,589)	(7,406)
Other income (expense)		(71)	64	29
Gain (loss) on sale/impairment		0	4,068	0
Net income (loss)		2,851	5,938	964
Company's equity in earnings (loss) of unconsolidated joint ventures		2,118	2,065	807
Liberty Washington, LP [Member]
Assets
Real estate assets		274,847	289,627
Accumulated depreciation		(49,106)	(44,235)
Real estate assets, net		225,741	245,392
Development in progress		0	0
Land held for development		0	0
Other assets		18,566	18,749
Total assets		244,307	264,141
Liabilities and Equity
Debt		102,624	103,667
Other liabilities		4,954	4,385
Equity		136,729	156,089
Total liabilities and equity		244,307	264,141
Company's net investment in unconsolidated joint ventures	[1]	34,107	38,863
Condensed Statements of Operations
Total revenue		25,439	39,727	71,101
Operating expense		9,147	15,935	27,384
Operating income (loss)		16,292	23,792	43,717
Interest		(4,065)	(10,866)	(17,353)
Depreciation and amortization		(5,926)	(10,750)	(20,725)
Other income (expense)		(65)	257	531
Gain (loss) on sale/impairment		(15,910)	40,943	(56,792)
Net income (loss)		(9,674)	43,376	(50,622)
Company's equity in earnings (loss) of unconsolidated joint ventures		(1,941)	10,857	(7,314)
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Assets
Real estate assets		496,210	495,842
Accumulated depreciation		(138,943)	(125,014)
Real estate assets, net		357,267	370,828
Development in progress		0	0
Land held for development		0	0
Other assets		49,755	48,074
Total assets		407,022	418,902
Liabilities and Equity
Debt		305,223	309,475
Other liabilities		9,488	9,964
Equity		92,311	99,463
Total liabilities and equity		407,022	418,902
Company's net investment in unconsolidated joint ventures	[1]	17,307	18,688
Condensed Statements of Operations
Total revenue		65,372	65,587	68,444
Operating expense		26,474	26,761	29,692
Operating income (loss)		38,898	38,826	38,752
Interest		(19,411)	(19,737)	(19,936)
Depreciation and amortization		(14,485)	(14,513)	(14,442)
Other income (expense)		(325)	(213)	(227)
Gain (loss) on sale/impairment		0	0	0
Net income (loss)		4,677	4,363	4,147
Company's equity in earnings (loss) of unconsolidated joint ventures		1,659	1,594	2,053
Liberty Property 18th & Arch [Member]
Assets
Real estate assets	[2]	551,874	0
Accumulated depreciation	[2]	(4,070)	0
Real estate assets, net	[2]	547,804	0
Development in progress	[2]	289,767	671,032
Land held for development	[2]	0	0
Other assets	[2]	98,191	52,053
Total assets	[2]	935,762	723,085
Liabilities and Equity
Debt	[2]	0	0
Other liabilities	[2]	102,982	95,574
Equity	[2]	832,780	627,511
Total liabilities and equity	[2]	935,762	723,085
Company's net investment in unconsolidated joint ventures	[1],[2]	177,843	132,952
Condensed Statements of Operations
Total revenue	[2]	16,544	0	0
Operating expense	[2]	3,417	280	221
Operating income (loss)	[2]	13,127	(280)	(221)
Interest	[2]	0	0	0
Depreciation and amortization	[2]	(4,077)	0	0
Other income (expense)	[2]	(547)	1,270	30
Gain (loss) on sale/impairment	[2]	30	0	0
Net income (loss)	[2]	8,533	990	(191)
Company's equity in earnings (loss) of unconsolidated joint ventures	[2]	1,782	213	(29)
Other Joint Ventures [Member]
Assets
Real estate assets		98,589	71,722
Accumulated depreciation		(10,995)	(9,702)
Real estate assets, net		87,594	62,020
Development in progress		0	0
Land held for development		41,075	42,259
Other assets		24,263	29,654
Total assets		152,932	133,933
Liabilities and Equity
Debt		63,999	57,660
Other liabilities		8,420	17,155
Equity		80,513	59,118
Total liabilities and equity		152,932	133,933
Company's net investment in unconsolidated joint ventures	[1]	27,694	22,712
Condensed Statements of Operations
Total revenue	[3]	11,118	9,888	9,387
Operating expense	[3]	3,365	2,801	2,727
Operating income (loss)	[3]	7,753	7,087	6,660
Interest	[3]	(2,807)	(2,462)	(2,248)
Depreciation and amortization	[3]	(2,124)	(1,925)	(1,840)
Other income (expense)	[3]	15,549	7,123	9,925
Gain (loss) on sale/impairment	[3]	0	(11)	0
Net income (loss)	[3]	18,371	9,812	12,497
Company's equity in earnings (loss) of unconsolidated joint ventures	[3]	9,963	4,634	6,517
Cambridge Medipark Ltd [Member] | Leaseholds and Leasehold Improvements [Member]
Condensed Statements of Operations
Gain (loss) on sale/impairment		$ 16,000	$ 7,100	$ 9,900

[1] Differences between the Company's net investment in unconsolidated joint ventures and its underlying equity in the net assets of the venture are primarily a result of impairments related to the Company's investment in unconsolidated joint ventures, the deferral of gains associated with the sales of properties to joint ventures in which the Company retains an ownership interest and loans made to the joint ventures by the Company. These adjustments have resulted in an aggregate difference reducing the Company's investments in unconsolidated joint ventures by $3.0 million and $1.4 million as of December 31, 2017 and 2016, respectively. Differences between historical cost basis and the basis reflected at the joint venture level (other than loans) are typically depreciated over the life of the related asset.

[2] Represents the combined results of two joint ventures related to the property at 18th and Arch Streets, Philadelphia.

[3] Other income/(expense) for this group of joint ventures reflects gains related to the sales of land leasehold interests totaling $16.0 million, $7.1 million and $9.9 million for the years ended December 31, 2017, 2016 and 2015, respectively.