Unconsolidated real estate joint ventures (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 340,190	$ 237,507
Unconsolidated Real Estate Joint Ventures Debt
Weighted-Average Interest Rate at End of Period	3.93%
Long-term Debt	$ 6,737,683
Equity Method Investee
Unconsolidated Real Estate Joint Ventures Debt
Long-term Debt	501,972
Long-term Debt, Remaining Commitments	215,181
Equity Method Investee \| Menlo Gateway
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 282,280	186,504
Our ownership percentage (in percent)	48.30%
Equity Method Investee \| 1401/1413 Research Boulevard
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 7,798	8,197
Our ownership percentage (in percent)	65.00%
Equity Method Investee \| 704 Quince Orchard Road
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 4,679	4,547
Our ownership percentage (in percent)	56.80%
Equity Method Investee \| 1655 and 1725 Third Street
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 36,508	34,917
Our ownership percentage (in percent)	10.00%
Equity Method Investee \| Other unconsolidated real estate joint ventures
Schedule of Equity Method Investments
Investments in unconsolidated real estate joint ventures	$ 8,925	$ 3,342
Secured debt maturing on 5/17/20 \| Equity Method Investee \| 1401/1413 Research Boulevard
Unconsolidated Real Estate Joint Ventures Debt
Maturity Date	May 17, 2020
Weighted-Average Interest Rate at End of Period	5.60%
Long-term Debt	$ 25,467
Long-term Debt, Percentage Bearing Variable Interest, Remaining Commitments	$ 3,268
Secured debt maturing on 6/29/21 \| Equity Method Investee \| 1655 and 1725 Third Street
Unconsolidated Real Estate Joint Ventures Debt
Maturity Date	Jun. 29, 2021
Weighted-Average Interest Rate at End of Period	5.80%
Long-term Debt	$ 282,513
Long-term Debt, Percentage Bearing Variable Interest, Remaining Commitments	$ 92,487
Secured debt maturing on 3/16/23 \| Equity Method Investee \| 704 Quince Orchard Road
Unconsolidated Real Estate Joint Ventures Debt
Maturity Date	Mar. 16, 2023
Weighted-Average Interest Rate at End of Period	4.23%
Long-term Debt	$ 7,571
Long-term Debt, Percentage Bearing Variable Interest, Remaining Commitments	$ 7,300
Secured debt maturing on 5/1/35 \| Equity Method Investee \| Menlo Gateway
Unconsolidated Real Estate Joint Ventures Debt
Maturity Date	May 01, 2035
Stated interest rate (as a percent)	4.53%
Weighted-Average Interest Rate at End of Period	4.59%
Long-term Debt	$ 43,700
Long-term Debt, Percentage Bearing Fixed Interest, Remaining Commitments	$ 112,126
Secured debt maturing on 8/10/35 \| Equity Method Investee \| Menlo Gateway
Unconsolidated Real Estate Joint Ventures Debt
Maturity Date	Aug. 10, 2035
Stated interest rate (as a percent)	4.15%
Weighted-Average Interest Rate at End of Period	4.18%
Long-term Debt	$ 142,721
Long-term Debt, Percentage Bearing Fixed Interest, Remaining Commitments	$ 0
London Interbank Offered Rate (LIBOR) \| Secured debt maturing on 5/17/20 \| Equity Method Investee \| 1401/1413 Research Boulevard
Unconsolidated Real Estate Joint Ventures Debt
Applicable margin (as a percent)	2.50%
London Interbank Offered Rate (LIBOR) \| Secured debt maturing on 6/29/21 \| Equity Method Investee \| 1655 and 1725 Third Street
Unconsolidated Real Estate Joint Ventures Debt
Applicable margin (as a percent)	3.70%
London Interbank Offered Rate (LIBOR) \| Secured debt maturing on 3/16/23 \| Equity Method Investee \| 704 Quince Orchard Road
Unconsolidated Real Estate Joint Ventures Debt
Applicable margin (as a percent)	1.95%

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