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