Investments (Details) - U.S. Treasury securities $ in Millions	12 Months Ended
	Dec. 31, 2017 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	$ 24.9
Total fair value	24.8
Unrealized loss	0.1
Other-than-temporary impairments	$ 0.0