Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (251,627)	$ (180,994)
Net gain (loss) during period	101,438	28,503
Ending balance	(150,189)	$ (152,491)
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	9,921
Net gain (loss) during period	(224)
Ending balance	9,697
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	38,166
Net gain (loss) during period	97,418
Ending balance	135,584
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(296,979)
Net gain (loss) during period	4,321
Ending balance	(292,658)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(2,735)
Net gain (loss) during period	(77)
Ending balance	$ (2,812)