Fair Value of Financial Instruments - Changes in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 258,413
Change in net realized gain (loss) on investments	$ 103	$ (58)	32	$ 140
Amounts reclassified for net realized gain (loss) included in net income	12	(38)	4	87
Other comprehensive income (loss), net	115	$ (96)	36	$ 227
Ending balance	327,641		327,641
Unrealized Gain (Loss) on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(156)
Change in net realized gain (loss) on investments			32
Amounts reclassified for net realized gain (loss) included in net income			4
Other comprehensive income (loss), net			36
Ending balance	$ (120)		$ (120)