Summary of significant accounting policies (Recent Accounting Pronouncements) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2019	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2018	Jan. 01, 2017	Dec. 31, 2016
New Accounting Pronouncement, Early Adoption
Restricted cash		$ 27,713		$ 27,713				$ 16,334
Revenue overall
Revenues subject to new accounting pronouncements				829,306
Sales of Real Estate				10,900
Leases
Ground Lease Obligation Capitalized Costs				18,400
Rent expense recognized under ground lease				9,400
Ground Lease Obligation Due		584,022		584,022
Financial instruments
Available-for-sale equity securities, cost basis		55,433		55,433				41,392
Net unrealized gain (loss)		45,189		45,189
Available-for-sale equity securities, at fair value		100,622		100,622				60,685
Investments accounted for under cost method		384,640		384,640				281,792
Investment Owned, at Cost		440,073		440,073
Investments unrealized gain (loss)				45,189
Investments		485,262		485,262				$ 342,477
Investment income				2,007
Unrealized holding gains (losses) arising during the period		17,018	$ (38,621)	23,414	$ (70,055)
Joint venture distribution
Distributions of earnings from unconsolidated real estate joint ventures				249	286
Return of capital from unconsolidated real estate joint ventures				38,576	$ 0
Accounting Standards Update 2016-02
Revenue overall
Revenues subject to new accounting pronouncements				793,444
Accounting Standards Update 2014-09
Revenue overall
Revenues subject to new accounting pronouncements				32,323
Gain on sale of real estate subject to new accounting pronouncements				381
Other accounting standards update
Revenue overall
Revenues subject to new accounting pronouncements				3,539
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2016-09 | Retained Earnings
New Accounting Pronouncement, Early Adoption
Cumulative Effect of New Accounting Principle in Period of Adoption							$ 368
Greater Boston market | 360 Longwood Avenue
Joint venture distribution
Distributions of earnings from unconsolidated real estate joint ventures				38,800
Rental revenues | Accounting Standards Update 2016-02
Revenue overall
Revenues subject to new accounting pronouncements				604,570
Tenant recoveries | Accounting Standards Update 2016-02
Revenue overall
Revenues subject to new accounting pronouncements				188,874
Investment Income | Accounting Standards Update 2016-01
Revenue overall
Revenues subject to new accounting pronouncements				2,007
Other Income | Other accounting standards update
Revenue overall
Revenues subject to new accounting pronouncements				1,532
Limited partnerships
Financial instruments
Investments accounted for under cost method		136,044		136,044
Net unrealized gain (loss) on cost method investments				0
Investments accounted for under cost method		136,044		136,044
Unrealized Gain (Loss) on Investments				71,800
Change in unrealized gain (loss) in privately held investments				16,200
Private Equity Funds
Financial instruments
Investments accounted for under cost method		248,596		248,596
Net unrealized gain (loss) on cost method investments				0
Investments accounted for under cost method		$ 248,596		$ 248,596
Scenario, Forecast | Accounting Standards Update 2016-02
Revenue overall
Date of new accounting pronouncement adoption	Jan. 01, 2019
Scenario, Forecast | Accounting Standards Update 2014-09
Revenue overall
Date of new accounting pronouncement adoption						Jan. 01, 2018
Scenario, Forecast | Accounting Standards Update 2016-01
Revenue overall
Date of new accounting pronouncement adoption						Jan. 01, 2018