Secured and unsecured senior debt Narratives (Details) (USD $)	12 Months Ended			1 Months Ended		7 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended				1 Months Ended				12 Months Ended	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Aug. 30, 2013 $1.5 billion unsecured senior line of credit	Apr. 30, 2012 $1.5 billion unsecured senior line of credit	Jul. 31, 2013 $1.5 billion unsecured senior line of credit	Dec. 31, 2013 $1.5 billion unsecured senior line of credit		Jul. 26, 2013 $1.5 billion unsecured senior line of credit	Feb. 29, 2012 Senior Notes 4.60% unsecured senior notes payable	Dec. 31, 2013 Senior Notes 2016 Unsecured Senior Bank Term Loan		Feb. 29, 2012 4.60% unsecured senior notes payable	Dec. 31, 2013 4.60% unsecured senior notes payable		Jul. 26, 2013 2016 Unsecured Senior Bank Term Loan	Aug. 30, 2013 2016 Unsecured Senior Bank Term Loan	Dec. 31, 2013 2016 Unsecured Senior Bank Term Loan		Jun. 30, 2012 Construction Loans Secured notes payable maturing on 7/1/15	Dec. 31, 2013 Federal Funds Rate $1.5 billion unsecured senior line of credit	Dec. 31, 2013 Eurodollar $1.5 billion unsecured senior line of credit	Dec. 31, 2013 LIBOR $1.5 billion unsecured senior line of credit	Jun. 30, 2012 LIBOR Construction Loans Secured notes payable maturing on 7/1/15	Jun. 30, 2012 Base Rate Construction Loans Secured notes payable maturing on 7/1/15	Dec. 31, 2012 Unsecured Debt	Dec. 31, 2012 Unsecured Bank Term Loan 2012	Dec. 31, 2011 Unsecured Bank Term Loan 2012	Dec. 31, 2012 Unsecured Senior Line of Credit and Bank Term Loans	May 31, 2013 San Francisco Bay Area Construction Loans Secured notes payable maturing on 06/1/16	Dec. 31, 2013 Greater Boston market Equity Method Investee	Dec. 31, 2012 Greater Boston market Equity Method Investee	Dec. 31, 2013 Greater Boston market Equity Method Investee Construction Loans	Dec. 31, 2013 Greater Boston market LIBOR Equity Method Investee Construction Loans Extension_Option	May 31, 2013 269 East Grand Avenue San Francisco Bay Area Construction Loans Secured notes payable maturing on 06/1/16	May 31, 2013 269 East Grand Avenue San Francisco Bay Area LIBOR Construction Loans Secured notes payable maturing on 06/1/16	May 31, 2013 269 East Grand Avenue San Francisco Bay Area Base Rate Construction Loans Secured notes payable maturing on 06/1/16	Aug. 31, 2013 75/125 Binney Street Greater Boston market Construction Loans Secured notes payable maturing on 8/23/17	Aug. 31, 2013 75/125 Binney Street Greater Boston market LIBOR Construction Loans Secured notes payable maturing on 8/23/17	Aug. 31, 2013 75/125 Binney Street Greater Boston market Base Rate Construction Loans Secured notes payable maturing on 8/23/17
Debt Instrument [Line Items]
Public offering of senior notes payable										$ 550,000,000.0	$ 500,000,000
Stated interest rate (as a percent)										4.60%	3.90%			4.60%														3.70%
Price as a percentage of principal amount of notes payable										99.915%	99.712%
Yield rate (as a percent)										4.61%	3.94%
Maturity date							Jan. 03, 2019	[1]			Jun. 15, 2023	[1]		Apr. 01, 2022	[1]			Jul. 31, 2016	[1]
Repayment of outstanding principal	250,000,000	250,000,000	500,000,000								150,000,000							250,000,000
Principal amount	3,061,061,000						204,000,000			250,000,000				549,603,000				500,000,000
Loss on early extinguishment of debt	1,992,000	2,225,000	6,485,000								560,000															2,225,000		6,485,000
Ownership interest in subsidiary (as a percent)	100.00%												100.00%
Maximum borrowing capacity	1,500,000,000.0								1,500,000,000.0
Write-off of unamortized loan fees																											600,000		1,600,000
Early extinguishment of debt																											250,000,000	217,100,000
Base rate				LIBOR		LIBOR	LIBOR	[2]								LIBOR	LIBOR	LIBOR			federal funds rate	Eurocurrency Rate		LIBOR	Base Rate									LIBOR		LIBOR	Base Rate		LIBOR	Base Rate
Applicable margin (as a percent)						1.20%	1.10%	[2]								1.75%		1.20%			0.50%	1.00%	1.10%	1.50%	0.25%									3.75%		1.40%	0.25%		1.35%	0.35%
Annual facility fee (as a percent)				0.20%	0.20%	0.25%	0.20%
Aggregate commitments of construction loan																				55,000,000													213,200,000		36,000,000.0
Pre-Leased percentage of property																																			100.00%
Debt instrument maturity extension period																				1 year										1 year								1 year
Principal amount	3,062,837,000						204,000,000							550,000,000				500,000,000						46,013,000
Number of one-year extension options																																		2
Aggregate commitment available for borrowing																				9,000,000															36,000,000			250,400,000
Percent of commitments available for borrowing																																						100.00%
Equity interest percentage (in percent)																															27.50%	55.00%
Drawn and outstanding construction loan																																	$ 87,900,000

[1] Includes any extension options that we control.

[2] In addition to the stated rate, the line of credit is subject to an annual facility fee of 0.20%.