Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ 16,173,289	$ 12,335,896
Net gain (loss) during period	$ (13,538)	$ 54,029	137,068	17,521
Ending balance	16,340,976	12,921,934	16,340,976	12,921,934
Investment securities With OTTI [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			9,921
Net gain (loss) during period			745
Ending balance	10,666		10,666
Investment securities All Other [Member] | Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			38,166
Net gain (loss) during period			126,586
Ending balance	164,752		164,752
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(296,979)
Net gain (loss) during period			11,654
Ending balance	(285,325)		(285,325)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(2,735)
Net gain (loss) during period			(1,917)
Ending balance	(4,652)		(4,652)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			(251,627)	(180,994)
Net gain (loss) during period			137,068
Ending balance	$ (114,559)	$ (163,473)	$ (114,559)	$ (163,473)