Available-for-Sale Securities - Additional Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) item
Available-for-Sale Securities
Contractual maturity period, maximum			1 year
Number of investments classified as available-for-sale securities in an unrealized loss position (in investments) | item	24		24		34
Number of investments classified as available-for-sale securities in an unrealized loss position for more than twelve months (in investments) | item	0		0		0
Aggregate fair value of securities none of which had been in an unrealized loss position for more than twelve months | $	$ 90,800		$ 90,800		$ 111,300
Proceeds from sales of available-for-sale securities | $	$ 0	$ 0	$ 0	$ 0