Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of changes in accumulated other comprehensive loss
Beginning Balance			$ 592,584
Total other comprehensive income (loss), net of tax	$ 311	$ (247)	1,159	$ (3,698)
Ending Balance	534,102		534,102
Unrealized (loss) gain on investments
Summary of changes in accumulated other comprehensive loss
Beginning Balance			(1,461)
Other comprehensive income before reclassifications			(1,295)
Amounts reclassified from accumulated other comprehensive loss			2,454
Total other comprehensive income (loss), net of tax	311	$ (247)	1,159	$ (3,698)
Ending Balance	$ (302)		$ (302)