Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (AOCI) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 456,150	$ 453,981	$ 446,968
Net current period other comprehensive income (loss)	4,104	(3,079)	(223)
Ending Balance	425,423	456,150	453,981
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(492)	2,587	2,810
Other comprehensive income (loss) before reclassification	4,823	(3,231)	(1,584)
Amounts reclassified from accumulated other comprehensive income (loss)	(719)	152	1,361
Net current period other comprehensive income (loss)	4,104	(3,079)	(223)
Ending Balance	$ 3,612	$ (492)	$ 2,587