Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of changes in accumulated other comprehensive loss
Beginning Balance	$ 592,584
Total other comprehensive income (loss), net of tax	787	$ (3,968)	$ 3,193
Ending Balance	533,111	592,584
Unrealized (loss) gain on investments
Summary of changes in accumulated other comprehensive loss
Beginning Balance	(1,461)	2,507
Other comprehensive income before reclassifications	(2,127)	1,235
Amounts reclassified from accumulated other comprehensive income (loss)	2,914	(5,203)
Total other comprehensive income (loss), net of tax	787	(3,968)
Ending Balance	$ (674)	$ (1,461)	$ 2,507