Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (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) [Roll Forward]
Beginning Balance			$ (24,623)	$ 113,863
Other comprehensive income (loss) before reclassifications			74,136	175,041
Amounts reclassified from accumulated other comprehensive income (loss)			1,797	(7,547)
Other comprehensive income (loss), net of tax	$ 57,160	$ 102,038	75,933	167,494
Ending Balance	51,310	281,357	51,310	281,357
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			16,153	160,611
Other comprehensive income (loss) before reclassifications			74,136	175,601
Amounts reclassified from accumulated other comprehensive income (loss)			32	(9,687)
Other comprehensive income (loss), net of tax			74,168	165,914
Ending Balance	90,321	326,525	90,321	326,525
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(40,776)	(46,748)
Other comprehensive income (loss) before reclassifications			0	(560)
Amounts reclassified from accumulated other comprehensive income (loss)			1,765	2,140
Other comprehensive income (loss), net of tax			1,765	1,580
Ending Balance	$ (39,011)	$ (45,168)	$ (39,011)	$ (45,168)