Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 425,423
Ending Balance	432,068
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	3,612	$ (492)
Other comprehensive income (loss) before reclassification	(5,634)	978
Amounts reclassified from accumulated other comprehensive income	(214)	(26)
Net current period other comprehensive income (loss)	(5,848)	952
Ending Balance	$ (2,236)	$ 460