Fair Value of Financial Instruments - Changes in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 343,544
Change in net unrealized loss on investments, net of tax	(407)	$ (62)
Amounts reclassified for net realized gain included in net income, net of tax	16	$ (8)
Ending balance	366,319
Unrealized Gain (Loss) on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(574)
Change in net unrealized loss on investments, net of tax	(407)
Amounts reclassified for net realized gain included in net income, net of tax	16
Other comprehensive income (loss), net	(391)
Ending balance	$ (965)