Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			$ 456,150
Ending Balance	$ 481,392		481,392
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	460	$ 5,380	(492)	$ 2,587
Other comprehensive income before reclassification	2,038	3,286	3,094	5,986
Amounts reclassified from accumulated other comprehensive income	(82)	(357)	(186)	(264)
Net current period other comprehensive income	1,956	2,929	2,908	5,722
Ending Balance	$ 2,416	$ 8,309	$ 2,416	$ 8,309