Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of changes in accumulated other comprehensive loss
Beginning Balance			$ 592,584
Total other comprehensive income (loss), net of tax	$ 219	$ (4,163)	848	$ (3,451)
Ending Balance	549,540		549,540
Unrealized gain (loss) on investments
Summary of changes in accumulated other comprehensive loss
Beginning Balance			(1,461)
Other comprehensive income before reclassifications			(366)
Amounts reclassified from accumulated other comprehensive loss			1,214
Total other comprehensive income (loss), net of tax	219	$ (4,163)	848	$ (3,451)
Ending Balance	$ (613)		$ (613)