Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			$ 456,150
Ending Balance	$ 494,945		494,945
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	2,416	$ 8,309	(492)	$ 2,587
Other comprehensive income (loss) before reclassification	820	(1,589)	4,032	4,286
Amounts reclassified from accumulated other comprehensive income	(203)	(124)	(507)	(277)
Net current period other comprehensive income (loss)	617	(1,713)	3,525	4,009
Ending Balance	$ 3,033	$ 6,596	$ 3,033	$ 6,596