Secured and unsecured senior debt Detail of secured and unsecured senior debt (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended	12 Months Ended
		Jan. 31, 2016	Oct. 31, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					$ 3,496,813
Unhedged variable rate					468,982
Principal amount					$ 3,965,795
Effective rate (as a percent)					3.33%
Annual facility fee (as a percent)				0.20%
Future principal payments due on secured and unsecured debt
Repayments of Secured Debt					$ 89,815	$ 231,051	$ 36,219
Balloon payments					3,934,846
Principal amortization					30,949
2016
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					166,465
Unhedged variable rate					68,397
Principal amount					234,862
Future principal payments due on secured and unsecured debt
Balloon payments					229,795
Principal amortization					5,067
2017
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					3,521
Unhedged variable rate					239,893
Principal amount					243,414
Future principal payments due on secured and unsecured debt
Balloon payments					239,893
Principal amortization					3,521
2018
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					4,833
Unhedged variable rate					0
Principal amount					4,833
Future principal payments due on secured and unsecured debt
Balloon payments					0
Principal amortization					4,833
2019
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					705,474
Unhedged variable rate					160,692
Principal amount					866,166
Future principal payments due on secured and unsecured debt
Balloon payments					860,692
Principal amortization					5,474
2020
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					507,975
Unhedged variable rate					0
Principal amount					507,975
Future principal payments due on secured and unsecured debt
Balloon payments					503,979
Principal amortization					3,996
Thereafter
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					2,108,545
Unhedged variable rate					0
Principal amount					2,108,545
Future principal payments due on secured and unsecured debt
Balloon payments					2,100,487
Principal amortization					8,058
Construction Loans
Future principal payments due on secured and unsecured debt
Principal amount					341,981
Secured notes payable
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					400,235
Unhedged variable rate					417,982
Principal amount					$ 818,217
Stated interest rate (as a percent)					3.92%
Effective rate (as a percent)			5.73%		3.84%
Future principal payments due on secured and unsecured debt
2016					$ 235,290
2017					243,858
2018					5,294
2019					115,645
2020					108,314
Thereafter					$ 109,816
Repayments of Secured Debt			$ 76,000
CMBS maturing on 4/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					5.82%
Effective rate (as a percent)					5.82%
Future principal payments due on secured and unsecured debt
2016					$ 29,471
2017					0
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 29,471
CMBS maturing on 1/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					5.74%
Effective rate (as a percent)					3.00%
Future principal payments due on secured and unsecured debt
2016					$ 6,916
2017					0
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 6,916
Secured notes payable maturing on 06/1/16
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)					1.83%
Future principal payments due on secured and unsecured debt
2016					$ 20,800
2017					0
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 20,800
Secured notes payable maturing on 7/1/16
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)					1.93%
Future principal payments due on secured and unsecured debt
2016					$ 47,596
2017					0
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 47,596
CMBS maturing on 8/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					6.35%
Effective rate (as a percent)					6.35%
Maturity date					Aug. 01, 2016
Future principal payments due on secured and unsecured debt
2016					$ 126,715
2017					0
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 126,715
Secured notes payable maturing on 1/20/17
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					2.42%
Effective rate (as a percent)					2.42%
Maturity date					Jan. 20, 2017
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					76,000
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 76,000
Secured notes payable maturing on 8/23/17
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)					1.77%
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					163,893
2018					0
2019					0
2020					0
Thereafter					0
Principal amount					$ 163,893
Secured notes payable maturing on 1/28/19
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)					1.83%
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					109,692
2020					0
Thereafter					0
Principal amount					$ 109,692
Secured notes payable maturing on 4/1/20
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					7.75%
Effective rate (as a percent)					7.75%
Maturity date					Apr. 01, 2020
Future principal payments due on secured and unsecured debt
2016					$ 1,696
2017					1,832
2018					1,979
2019					2,138
2020					104,352
Thereafter					0
Principal amount					$ 111,997
Secured notes payable maturing on 1/1/23
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					4.66%
Effective rate (as a percent)					4.66%
Maturity date					Jan. 01, 2023
Future principal payments due on secured and unsecured debt
2016					$ 1,466
2017					1,540
2018					1,614
2019					1,692
2020					1,770
Thereafter					29,904
Principal amount					$ 37,986
Secured notes payable maturing on 3/10/23
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					3.93%
Effective rate (as a percent)					3.10%
Maturity date					Mar. 10, 2023
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					1,091
2019					1,505
2020					1,566
Thereafter					77,838
Principal amount					$ 82,000
Secured notes payable maturing on 7/1/36
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					6.50%
Effective rate (as a percent)					6.50%
Maturity date					Jul. 01, 2036
Future principal payments due on secured and unsecured debt
2016					$ 20
2017					20
2018					22
2019					23
2020					25
Thereafter					703
Principal amount					813
Unamortized Premiums
Future principal payments due on secured and unsecured debt
Unamortized Premium					4,338
Unamortized Premiums | 2016
Future principal payments due on secured and unsecured debt
Unamortized Premium					610
Unamortized Premiums | 2017
Future principal payments due on secured and unsecured debt
Unamortized Premium					573
Unamortized Premiums | 2018
Future principal payments due on secured and unsecured debt
Unamortized Premium					588
Unamortized Premiums | 2019
Future principal payments due on secured and unsecured debt
Unamortized Premium					595
Unamortized Premiums | 2020
Future principal payments due on secured and unsecured debt
Unamortized Premium					601
Unamortized Premiums | Thereafter
Future principal payments due on secured and unsecured debt
Unamortized Premium					1,371
Unsecured Debt
Summary of fixed rate/hedged and floating rate debt
Principal amount					$ 3,147,578
Effective rate (as a percent)					3.20%
Unsecured Debt | 2016
Future principal payments due on secured and unsecured debt
2016					$ (428)
Unsecured Debt | 2017
Future principal payments due on secured and unsecured debt
2017					(444)
Unsecured Debt | 2018
Future principal payments due on secured and unsecured debt
2018					(461)
Unsecured Debt | 2019
Future principal payments due on secured and unsecured debt
2019					750,521
Unsecured Debt | 2020
Future principal payments due on secured and unsecured debt
2020					399,661
Unsecured Debt | Thereafter
Future principal payments due on secured and unsecured debt
Thereafter					1,998,729
Line of Credit
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					100,000
Unhedged variable rate					51,000
Principal amount					$ 151,000
Effective rate (as a percent)					1.62%
Maturity date					Jan. 03, 2019
Annual facility fee (as a percent)					0.20%
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					151,000
2020					0
Thereafter					0
Principal amount					151,000
2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					600,000
Unhedged variable rate					0
Principal amount					$ 600,000
Effective rate (as a percent)					1.71%
Maturity date					Jan. 03, 2019
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					600,000
2020					0
Thereafter					0
Principal amount					600,000
2021 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Fixed rate/hedged variable rate					350,000
Unhedged variable rate					0
Principal amount					$ 350,000
Effective rate (as a percent)					1.52%
Maturity date					Jan. 15, 2021
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					0
Thereafter					350,000
Principal amount					$ 350,000
2.75% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					2.75%
Effective rate (as a percent)					2.79%
Maturity date					Jan. 15, 2020
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					400,000
Thereafter					0
Principal amount					$ 400,000
4.60% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					4.60%
Effective rate (as a percent)					4.61%
Maturity date					Apr. 01, 2022
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					0
Thereafter					550,000
Principal amount					$ 550,000
3.90% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					3.90%
Effective rate (as a percent)					3.94%
Maturity date					Jun. 15, 2023
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					0
Thereafter					500,000
Principal amount					$ 500,000
4.30% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					4.30%
Effective rate (as a percent)					4.35%
Maturity date					Jan. 15, 2026
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					0
Thereafter					300,000
Principal amount					$ 300,000
4.50% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)					4.50%
Effective rate (as a percent)					4.51%
Maturity date					Jul. 30, 2029
Future principal payments due on secured and unsecured debt
2016					$ 0
2017					0
2018					0
2019					0
2020					0
Thereafter					300,000
Principal amount					300,000
Unamortized Discounts
Future principal payments due on secured and unsecured debt
Unamortized Discount					(3,422)
Unamortized Discounts | 2016
Future principal payments due on secured and unsecured debt
Unamortized Discount					(428)
Unamortized Discounts | 2017
Future principal payments due on secured and unsecured debt
Unamortized Discount					(444)
Unamortized Discounts | 2018
Future principal payments due on secured and unsecured debt
Unamortized Discount					(461)
Unamortized Discounts | 2019
Future principal payments due on secured and unsecured debt
Unamortized Discount					(479)
Unamortized Discounts | 2020
Future principal payments due on secured and unsecured debt
Unamortized Discount					(339)
Unamortized Discounts | Thereafter
Future principal payments due on secured and unsecured debt
Unamortized Discount					$ (1,271)
LIBOR | Secured notes payable maturing on 06/1/16
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.40%
LIBOR | Secured notes payable maturing on 7/1/16
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.50%
LIBOR | Secured notes payable maturing on 8/23/17
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.35%
LIBOR | Secured notes payable maturing on 1/28/19
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.50%
LIBOR | Line of Credit
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)	[1]				1.10%
LIBOR | 2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.20%
LIBOR | 2021 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Basis spread on LIBOR (as a percent)					1.10%
Secured notes payable maturing on 1/28/19 | Greater Boston | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Maturity date					Jan. 28, 2019
Future principal payments due on secured and unsecured debt
Principal amount					$ 109,692
Secured notes payable maturing on 8/23/17 | Greater Boston | Construction Loans
Summary of fixed rate/hedged and floating rate debt
Maturity date					Aug. 23, 2017
Future principal payments due on secured and unsecured debt
Principal amount					$ 163,893
Subsequent Event | Secured notes payable
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)		5.28%
Future principal payments due on secured and unsecured debt
Repayments of Secured Debt		$ 36,400

[1] (3)In January 2016, we repaid two secured notes payable aggregating $36.4 million with a weighted average effective interest rate of 5.28%.