Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) segment fund subsidiary	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Real Estate Properties [Line Items]
Development costs capitalized	$ 66,000,000	$ 31,600,000	$ 3,700,000
Interest costs capitalized	2,700,000	1,200,000	940,000
Additional basis	2,900,000	2,900,000
Impairment of long-lived assets	0	0	0
Straight-line rent recognized	12,855,000	13,599,000	4,840,000
Lease termination revenue	2,100,000	2,400,000	2,200,000
Revenue recognized for leasehold improvements	$ 2,600,000	$ 2,600,000	$ 1,900,000
Number of reportable business segments | segment	2
Percentage of minimum distribution of taxable income to qualify as a REIT (at least)	90.00%
Number of subsidiaries elected to be treated as taxable REIT subsidiaries | subsidiary	2
Partnership X [Member]
Real Estate Properties [Line Items]
Equity interest of the Fund, percent	24.30%
Fund X [Member]
Real Estate Properties [Line Items]
Equity interest of the Fund, percent	69.40%
Opportunity Fund [Member]
Real Estate Properties [Line Items]
Equity interest of the Fund, percent	6.20%
Unconsolidated Properties [Member]
Real Estate Properties [Line Items]
Number of real estate funds owned and managed | fund	3
Building [Member]
Real Estate Properties [Line Items]
Estimated useful life	40 years
Site Improvements [Member]
Real Estate Properties [Line Items]
Estimated useful life	15 years