Available for Sale Securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Investments Debt And Equity Securities [Abstract]
Gains (losses) on available for sale securities	$ 0	$ 0
Available-for-sale securities, unrealized loss position for more than12 months	9,000	$ 0
Available-for-sale securities, unrealized loss position fair value for more than12 months	$ 4,000,000
Available-for-sale, securities in unrealized loss positions,number of securities | Security	57	27
Available-for-sale securities, unrealized loss position for less than12 months	$ 433,000	$ 40,000
Available-for-sale securities, unrealized loss position fair value for less than12 months	$ 229,500,000	$ 60,300,000