Available-for-Sale Securities - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) item	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) item
Investments, Debt and Equity Securities [Abstract]
Contractual maturity period, maximum	1 year
Number of investments classified as available-for-sale securities in an unrealized loss position (in investments) | item	44		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
Aggregate fair value of securities none of which had been in an unrealized loss position for more than twelve months | $	$ 155,300		$ 111,300
Proceeds from sales of available-for-sale securities | $	$ 0	$ 0