Investments (Details) - U.S. Treasury securities $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized loss position | security	1
Other-than-temporary impairments	$ 0
Total amortized cost	5,000	$ 24,900
Total fair value	5,000	24,800
Unrealized loss	$ 1	$ 96