Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 0
Unrealized loss position on aggregate fair value of investments	0		$ 2,998,000
Gross realized gains or losses from sale or maturity of marketable securities	0	$ 0
Gross interest income on securities	66,000	65,000
Amortization expense on securities	$ 2,000	$ 8,000