Significant Accounting Policies - Investment in Marketable Securities (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investment in Marketable Securities
Marketable securities available-for-sale	$ 24,355,000		$ 24,355,000		$ 28,570,000
Equity marketable securities	34,428,000		34,428,000		28,570,000
Marketable securities, unrealized gain	2,000,000		100,000
Proceeds from sale of marketable securities	0	$ 0	$ 4,500,000	$ 0
Number of marketable securities sold | security			1
Fair Value
Investment in Marketable Securities
Marketable securities available-for-sale	24,355,000		$ 24,355,000		28,570,000
Equity marketable securities	34,428,000		34,428,000		28,570,000
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities available-for-sale	24,355,000		24,355,000		28,570,000
Cost basis of commercial mortgage-backed securities	$ 23,000,000		$ 23,000,000		27,500,000
Number of securities in continuous unrealized loss position | security	1		1
Unrealized loss			$ 500,000		700,000
Fair value of security in a continuous unrealized loss position for less than 12 months	$ 7,200,000		7,200,000		7,000,000
Equity Securities
Investment in Marketable Securities
Equity marketable securities	$ 10,073,000		$ 10,073,000		$ 0