Secured and unsecured senior debt Detail of secured and unsecured debt (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Extension_Option	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument
Effective rate (as a percent)	3.77%
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 5,224,496
Unamortized (Deferred Financing Cost), (Discount)/Premium	(13,698)
Total Consolidated	$ 5,210,798
Secured notes payable
Debt Instrument
Stated interest rate (as a percent)	4.51%
Effective rate (as a percent)	4.08%
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 759,496
Unamortized (Deferred Financing Cost), (Discount)/Premium	16,193
Total Consolidated	$ 775,689
Secured Notes Payable Maturing on 4/1/20
Debt Instrument
Stated interest rate (as a percent)	7.75%
Effective rate (as a percent)	8.12%
Maturity Date	Apr. 01, 2020
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 107,989
Unamortized (Deferred Financing Cost), (Discount)/Premium	(668)
Total Consolidated	$ 107,321
Secured Notes Payable Maturing on 1/1/23
Debt Instrument
Stated interest rate (as a percent)	4.66%
Effective rate (as a percent)	4.90%
Maturity Date	Jan. 01, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 34,579
Unamortized (Deferred Financing Cost), (Discount)/Premium	(313)
Total Consolidated	$ 34,266
Secured Notes Payable Maturing on 3/10/23
Debt Instrument
Stated interest rate (as a percent)	3.93%
Effective rate (as a percent)	3.19%
Maturity Date	Mar. 10, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 82,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	2,697
Total Consolidated	$ 84,697
Secured Notes Payable Maturing on 2/6/24
Debt Instrument
Stated interest rate (as a percent)	4.82%
Effective rate (as a percent)	3.39%
Maturity Date	Feb. 06, 2024
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 202,694
Unamortized (Deferred Financing Cost), (Discount)/Premium	15,475
Total Consolidated	$ 218,169
Secured Notes Payable Maturing on 7/1/36
Debt Instrument
Stated interest rate (as a percent)	6.50%
Effective rate (as a percent)	6.67%
Maturity Date	Jul. 01, 2036
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 773
Unamortized (Deferred Financing Cost), (Discount)/Premium	0
Total Consolidated	$ 773
Unsecured Debt
Debt Instrument
Effective rate (as a percent)	3.72%
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 4,465,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(29,891)
Total Consolidated	$ 4,435,109
2019 Unsecured Senior Bank Term Loan
Debt Instrument
Effective rate (as a percent)	2.77%
Maturity Date	Jan. 03, 2019
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 200,000		$ 200,000	$ 400,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(378)
Total Consolidated	$ 199,622
2021 Unsecured Senior Bank Term Loan
Debt Instrument
Effective rate (as a percent)	2.56%
Maturity Date	Jan. 15, 2021
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(1,425)
Total Consolidated	$ 348,575
$1.65 billion unsecured senior line of credit
Debt Instrument
Effective rate (as a percent)	2.53%
Maturity Date	Oct. 29, 2021
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 490,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	0
Total Consolidated	$ 490,000
2.75% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)	2.75%
Effective rate (as a percent)	2.96%
Maturity Date	Jan. 15, 2020
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 400,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(1,432)
Total Consolidated	$ 398,568
4.60% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)	4.60%
Effective rate (as a percent)	4.74%
Maturity Date	Apr. 01, 2022
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 550,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(2,600)
Total Consolidated	$ 547,400
3.90% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)	3.90%
Effective rate (as a percent)	4.04%
Maturity Date	Jun. 15, 2023
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 500,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(3,091)
Total Consolidated	$ 496,909
3.45% unsecured senior notes payable due in 2025
Debt Instrument
Stated interest rate (as a percent)	3.45%	3.45%
Effective rate (as a percent)	3.63%
Maturity Date	Apr. 30, 2025	Apr. 30, 2025
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 600,000	$ 600,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(6,167)
Total Consolidated	$ 593,833
4.30% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)	4.30%
Effective rate (as a percent)	4.51%
Maturity Date	Jan. 15, 2026
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(3,765)
Total Consolidated	$ 296,235
3.95% Unsecured Senior Notes Payable Due in 2027
Debt Instrument
Stated interest rate (as a percent)	3.95%
Effective rate (as a percent)	4.14%
Maturity Date	Jan. 15, 2027
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 350,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(4,398)
Total Consolidated	$ 345,602
3.95% unsecured senior notes payable due in 2028
Debt Instrument
Stated interest rate (as a percent)	3.95%		3.95%
Effective rate (as a percent)	4.09%
Maturity Date	Jan. 15, 2028		Jan. 15, 2028
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 425,000		$ 425,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(4,128)
Total Consolidated	$ 420,872
4.50% Unsecured Senior Notes Payable
Debt Instrument
Stated interest rate (as a percent)	4.50%
Effective rate (as a percent)	4.61%
Maturity Date	Jul. 30, 2029
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 300,000
Unamortized (Deferred Financing Cost), (Discount)/Premium	(2,507)
Total Consolidated	$ 297,493
London Interbank Offered Rate (LIBOR) | 2019 Unsecured Senior Bank Term Loan
Debt Instrument
Applicable margin (as a percent)	1.20%
London Interbank Offered Rate (LIBOR) | 2021 Unsecured Senior Bank Term Loan
Debt Instrument
Applicable margin (as a percent)	1.10%
London Interbank Offered Rate (LIBOR) | $1.65 billion unsecured senior line of credit
Debt Instrument
Applicable margin (as a percent)	1.00%
Secured Notes Payable Maturing on 1/28/19 | Construction Loans
Debt Instrument
Effective rate (as a percent)	3.36%
Maturity Date	Jan. 28, 2019
Total aggregate commitment	$ 350,000
Remaining Commitment	$ 18,539
Debt Instrument, Number of One-Year Maturity Date Extension Option | Extension_Option	2
Debt Instrument, Extended Maturity Period	1 year
Future principal payments due on secured and unsecured debt
Outstanding Balance	$ 331,461
Unamortized (Deferred Financing Cost), (Discount)/Premium	(998)
Total Consolidated	$ 330,463
Secured Notes Payable Maturing on 1/28/19 | London Interbank Offered Rate (LIBOR) | Construction Loans
Debt Instrument
Applicable margin (as a percent)	1.50%