Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 16,250,819	$ 16,486,622
Cumulative effect of change in accounting principle — equity securities	(16,853)
Net gain (loss) during period	(102,576)	3,069
Ending balance	15,709,694	16,213,104
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(44,150)
Cumulative effect of change in accounting principle — equity securities	(16,853)
Net gain (loss) during period	(100,684)
Ending balance	(161,687)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(304,546)
Net gain (loss) during period	7,260
Ending balance	(297,286)
Accumulated Other Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(15,118)
Net gain (loss) during period	(9,152)
Ending balance	(24,270)
Accumulated Other Comprehensive Income (Loss), Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(363,814)	(294,636)
Cumulative effect of change in accounting principle — equity securities	(16,853)
Net gain (loss) during period	(102,576)
Ending balance	$ (483,243)	$ (291,567)