Debt - Narrative (Details)		1 Months Ended	12 Months Ended
	Jul. 18, 2017 USD ($) extension_option	Oct. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Loss on extinguishment of debt			$ (701,000)	$ 0	$ 0
Interest costs incurred			65,400,000	54,300,000	55,500,000
Capitalized interest			12,727,000	$ 4,076,000	$ 6,437,000
Mortgages payable
Line of Credit Facility [Line Items]
Net carrying value of real estate collateralizing the mortgages payable			2,900,000,000
Proceeds from new debt			79,300,000
Repayment of mortgages payable			250,000,000
Ability to draw additional borrowing capacity			143,700,000
Loss on extinguishment of debt			(701,000)
Unsecured term loan, net			1,400,000,000
Line of credit
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	$ 1,400,000,000
Fees and expenses on debt	11,200,000
Line of credit | Revolving credit facility
Line of Credit Facility [Line Items]
Unsecured term loan, net	115,800,000
Credit facility, maximum borrowing capacity	$ 1,000,000,000
Number of debt extension options | extension_option	2
Debt extension option period (in months)	6 months
Line of credit | Tranche A-1 Term Loan
Line of Credit Facility [Line Items]
Unsecured term loan, net	$ 50,000,000	$ 50,000,000
Credit facility, maximum borrowing capacity	200,000,000
Line of credit | Tranche A-2 Term Loan
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	$ 200,000,000
Minimum | LIBOR | Line of credit | Revolving credit facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.10%
Minimum | LIBOR | Line of credit | Tranche A-1 Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.20%	1.97%
Minimum | LIBOR | Line of credit | Tranche A-2 Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.55%
Maximum | LIBOR | Line of credit | Revolving credit facility
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Maximum | LIBOR | Line of credit | Tranche A-1 Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.70%
Maximum | LIBOR | Line of credit | Tranche A-2 Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.35%
JBG Companies
Line of Credit Facility [Line Items]
Mortgages payable assumed	$ (768,523,000)
JBG Companies | Mortgages payable
Line of Credit Facility [Line Items]
Mortgages payable assumed	$ (768,500,000)
Repayment of mortgages payable			$ 64,800,000