Benefit Plans - 401(k) Savings Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Savings Plan
Maximum discretionary contribution matched by employer for each employee's contribution	$ 2,000	$ 2,000
Contribution expense	$ 535,000	$ 454,000	$ 342,000