Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive (Loss) Income, Net of Tax [Roll Forward]
Balance at beginning period	$ 592,584
Total other comprehensive income (loss), net of tax	629	$ 712
Balance at period end	581,196
Unrealized Gain and (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive (Loss) Income, Net of Tax [Roll Forward]
Balance at beginning period	(1,461)
Other comprehensive income before reclassifications	(606)
Amounts reclassified from accumulated other comprehensive loss	1,235
Total other comprehensive income (loss), net of tax	629
Balance at period end	$ (832)