Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net Consisted of Unrealized Gains (Losses) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (64,159)	$ (240,264)	$ (356,441)
Net gain (loss) during period	(116,835)	176,105	116,177
Ending balance	(180,994)	(64,159)	(240,264)
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	22,632	(55,790)	(84,029)
Net gain (loss) during period	(18,114)	78,422	28,239
Ending balance	4,518	22,632	(55,790)
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	11,294	92,581	5,995
Net gain (loss) during period	111,389	(81,287)	86,586
Ending balance	122,683	11,294	92,581
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(98,182)	(276,771)	(277,716)
Net gain (loss) during period	(207,407)	178,589	945
Ending balance	(305,589)	(98,182)	(276,771)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	97	(284)	(691)
Net gain (loss) during period	(2,703)	381	407
Ending balance	$ (2,606)	$ 97	$ (284)