Commitments And Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2009	May 19, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 10, 2014
Commitments And Contingencies [Line Items]
Letter of credit and performance obligations	$ 9.1
Property insurance program per occurrence limits	1,000.0
Per occurrence limit for NBCR Coverage	1,000.0
Value of program trigger	$ 160.0
Coinsurance of program trigger	18.00%
Deductible in insurance as a percentage of the value of the affected property, San Francisco and Los Angeles	3.00%
Per occurrence limit of the earthquake insurance which covers San Francisco and Los Angeles regions	$ 240.0
Annual aggregate limit of the earthquake insurance which covers San Francisco and Los Angeles regions	240.0
Amount of earthquake insurance provided by IXP, LLC as direct insurer San Francisco and Los Angeles	20.0
767 Venture, LLC [Member]
Commitments And Contingencies [Line Items]
Maximum funding obligation	171.7
Property insurance program per occurrence limits	1,625.0
Lehman [Member]
Commitments And Contingencies [Line Items]
Bankruptcy claim, amount filed by general creditor		$ 45.3
Bankruptcy claim amount allowed by court to creditor							$ 45.2
Bankruptcy Claims, Amount of Claims Settled			$ 0.4	$ 1.4	$ 8.1	$ 7.7
Bankruptcy remaining claim amount allowed by court to creditor	27.6
601 Lexington Avenue [Member]
Commitments And Contingencies [Line Items]
Coverage For Acts Of Terrorism Under TRIA Covered in Excess of Amount Covered by IXP	250.0
Boston Properties Limited Partnership
Commitments And Contingencies [Line Items]
Operating partnership guarantee to cover liabilities of IXP	$ 20.0