Investments - Schedule of Available-for-Sale Debt Securities that were in a Continuous Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2017 USD ($)
Gross Unrealized Losses
Less Than 12 Months	$ (31)
12 Months or Greater	(58)
Total	(89)
Estimated Fair Value
Less Than 12 Months	11,970
12 Months or Greater	11,939
Total	23,909
U.S. government agency securities
Gross Unrealized Losses
Less Than 12 Months	(31)
12 Months or Greater	(58)
Total	(89)
Estimated Fair Value
Less Than 12 Months	11,970
12 Months or Greater	11,939
Total	$ 23,909