Accumulated Other Comprehensive (Loss) Income - Summary of Changes in Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 686,799	$ 673,447	$ 979,701
Total other comprehensive (loss) income, net of tax	(3,968)	3,193	639
Ending Balance	592,584	686,799	673,447
Unrealized Gain and (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	2,507	(686)
Other comprehensive income before reclassifications	1,235	3,685
Amounts reclassified from accumulated other comprehensive loss	(5,203)	(492)
Total other comprehensive (loss) income, net of tax	(3,968)	3,193
Ending Balance	$ (1,461)	$ 2,507	$ (686)