Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (24,623)	$ 113,863
Other comprehensive income (loss) before reclassifications	17,891	75,143
Amounts reclassified from accumulated other comprehensive income (loss)	882	(9,687)
Other comprehensive income (loss), net of tax	18,773	65,456
Ending Balance	(5,850)	179,319
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	16,153	160,611
Other comprehensive income (loss) before reclassifications	17,891	74,134
Amounts reclassified from accumulated other comprehensive income (loss)	0	(9,687)
Other comprehensive income (loss), net of tax	17,891	64,447
Ending Balance	34,044	225,058
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(40,776)	(46,748)
Other comprehensive income (loss) before reclassifications	0	1,009
Amounts reclassified from accumulated other comprehensive income (loss)	882	0
Other comprehensive income (loss), net of tax	882	1,009
Ending Balance	$ (39,894)	$ (45,739)