Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (180,994)	$ (64,159)
Net gain (loss) during period	17,521	76,626
Ending balance	(163,473)	$ 12,467
Investment Securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	4,518
Net gain (loss) during period	5,926
Ending balance	10,444
Investment Securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	122,683
Net gain (loss) during period	(4,873)
Ending balance	117,810
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(305,589)
Net gain (loss) during period	16,166
Ending balance	(289,423)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(2,606)
Net gain (loss) during period	302
Ending balance	$ (2,304)