Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI		$ (152,373)	$ 2,855	$ (150,627)	$ (4,298)
Amortization of net actuarial loss				(15,792)
Amortization of prior service credit				3,268
Settlement costs				(152,388)
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains on sales of securities		928	8,687	14,285	13,707
Net other-than-temporary impairment losses					(485)
Reclassifications out of AOCI		928	8,687	14,285	13,222
Tax effect		(332)	(3,261)	(5,778)	(5,057)
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI		(153,301)	(5,832)	(164,912)	(17,520)
Tax effect		62,276	2,229	66,702	6,701
Amortization of net actuarial loss	[1]	(1,958)	(7,043)	(15,792)	(21,153)
Amortization of prior service credit	[1]	1,045	$ 1,211	3,268	$ 3,633
Settlement costs	[1]	$ (152,388)		$ (152,388)

[1] These components of AOCI are included in the computation of net periodic cost. See Note 9 Employee Benefit Plans for additional details.