Significant Accounting Policies - Investment in Marketable Securities (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) security	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investment in Marketable Securities
Marketable securities	$ 23,784,000		$ 28,570,000
Equity marketable securities	23,784,000		28,570,000
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	$ 23,784,000		28,570,000
Equity marketable securities
Investment in Marketable Securities
Equity marketable securities	0		0
Commercial mortgage-backed securities
Investment in Marketable Securities
Marketable securities	23,784,000		28,570,000
Cost basis of commercial mortgage-backed securities	$ 23,000,000.0		27,500,000
Number of securities in continuous unrealized loss position | security	1
Unrealized loss	$ 700,000		700,000
Fair value of security in a continuous unrealized loss position for less than 12 months	$ 7,100,000		$ 7,000,000.0