Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Marketable Securities [Line Items]
Investment owned, balance, shares	535,265	535,265
Economic basis per share (in dollars per share)	$ 56.05
Available-for-sale securities, amortized cost basis	$ 30,000,000
Fair value	$ 23,166,000	$ 35,156,000
Closing share price (in dollars per share)	$ 43.28	$ 65.68
Change in fair value of marketable securities (see Note 5)	$ (11,990,000)	$ 0	$ 0
Macerich
Marketable Securities [Line Items]
Closing share price (in dollars per share)	$ 43.28	$ 65.68
Change in fair value of marketable securities (see Note 5)	$ (11,990,000)