Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (180,994)	$ (64,159)	$ (240,264)
Net gain (loss) during period	(70,633)	(116,835)	176,105
Ending balance	(251,627)	(180,994)	(64,159)
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	4,518	22,632	(55,790)
Net gain (loss) during period	5,403	(18,114)	78,422
Ending balance	9,921	4,518	22,632
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	122,683	11,294	92,581
Net gain (loss) during period	(84,517)	111,389	(81,287)
Ending balance	38,166	122,683	11,294
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(305,589)	(98,182)	(276,771)
Net gain (loss) during period	8,610	(207,407)	178,589
Ending balance	(296,979)	(305,589)	(98,182)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(2,606)	97	(284)
Net gain (loss) during period	(129)	(2,703)	381
Ending balance	$ (2,735)	$ (2,606)	$ 97