Real Estate and Other Affiliates (Narrative) (Details)		3 Months Ended						9 Months Ended
	May 23, 2019 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2016	Mar. 31, 2015 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)
Investments in and Advances to Affiliates [Line Items]
Equity in earnings (losses) from real estate and other affiliates		$ 15,796,000		$ 11,349,000
Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Equity method investment		385,633,000							$ 373,192,000
Equity in earnings (losses) from real estate and other affiliates		12,041,000		7,625,000
Other land, rental and property revenues
Investments in and Advances to Affiliates [Line Items]
Revenues from contracts with customers		$ 23,243,000		34,897,000
USAA Joint Venture
Investments in and Advances to Affiliates [Line Items]
Contribution of property					$ 33,600,000
Fair value of property contributed					$ 85,000,000.0
Capital contribution to joint venture								$ 9,800,000
Preferred return on capital					9.00%
Ownership interest		90.00%
USAA Joint Venture | Local Developer
Investments in and Advances to Affiliates [Line Items]
Capital contribution to joint venture					$ 5,000,000.0			$ 1,100,000
USAA Joint Venture | USAA
Investments in and Advances to Affiliates [Line Items]
Capital contribution to joint venture	$ 178,400,000				64,000,000.0
Joint venture capital obligations	$ 8,800,000				$ 105,600,000
Preferred return on capital					9.00%
Preferred return on capital of cash distribution					11.11%
Ownership interest		10.00%
110 North Wacker
Investments in and Advances to Affiliates [Line Items]
Equity method investment			$ 273,600,000
110 North Wacker | Strategic Developments Segment
Investments in and Advances to Affiliates [Line Items]
Equity in earnings (losses) from real estate and other affiliates			267,500,000
110 North Wacker | Strategic Developments Segment | Other land, rental and property revenues | Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Revenues from contracts with customers			$ 15,400,000
Ssäm Bar
Investments in and Advances to Affiliates [Line Items]
Preferred return on capital of cash distribution		75.00%
Joint venture real estate, percentage funded						89.75%
Preferred return on capital		10.00%
Join venture real estate, remaining cash distribution percentage		50.00%
Ssäm Bar | Momofuku
Investments in and Advances to Affiliates [Line Items]
Preferred return on capital of cash distribution		25.00%
Joint venture real estate, percentage funded						10.25%
Ssäm Bar | Seaport Segment | Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Equity method investment		$ 7,302,000							$ 7,101,000
Equity in earnings (losses) from real estate and other affiliates		(352,000)		$ (1,392,000)
Investment in real estate and other affiliates		7,300,000
The Summit
Investments in and Advances to Affiliates [Line Items]
Preferred return on capital							5.00%
Cash distribution entitlement by the joint venture | item							2
The Summit | Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Contribution of property							$ 13,400,000
SID bonds transferred to joint venture							1,300,000
Transaction value of land contributed to joint venture							125,400,000
Transactional value per acre of land contributed to joint venture							226,000
Maximum contribution required to joint venture by co-venturer							$ 30,000,000.0
110 North Wacker
Investments in and Advances to Affiliates [Line Items]
Recourse amount		100,600,000
Unconsolidated Properties
Investments in and Advances to Affiliates [Line Items]
Secured debt real estate affiliates		584,000,000.0
Share of entity in secured debt		$ 286,900,000