Significant Accounting Policies - Investment in Marketable Securities (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) security	Mar. 31, 2021 USD ($)	Jun. 30, 2021 USD ($) security	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investment in Marketable Securities
Marketable securities available-for-sale	$ 24,289,000		$ 24,289,000		$ 28,570,000
Equity marketable securities	32,339,000		32,339,000		28,570,000
Marketable securities, unrealized loss	1,900,000		1,900,000
Proceeds from sale of marketable securities			$ 5,000,000.0	$ 0
Number of marketable securities sold | security			1
Fair Value
Investment in Marketable Securities
Marketable securities available-for-sale	24,289,000		$ 24,289,000		28,570,000
Equity marketable securities	32,339,000		32,339,000		28,570,000
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities available-for-sale	24,289,000		24,289,000		28,570,000
Cost basis of commercial mortgage-backed securities	$ 23,000,000.0		$ 23,000,000.0		27,500,000
Number of securities in continuous unrealized loss position | security	1		1
Unrealized loss		$ 700,000	$ 600,000
Fair value of security in a continuous unrealized loss position for less than 12 months	$ 7,100,000		7,100,000		7,000,000.0
Equity Securities
Investment in Marketable Securities
Equity marketable securities	$ 8,050,000		$ 8,050,000		$ 0