Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI		$ (2,682)	$ (152,373)	$ (5,237)	$ (150,627)
Amortization of net actuarial loss				(3,615)
Amortization of prior service credit				3,135
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of securities	[1]	(2,522)	928	(4,757)	14,285
Reclassifications out of AOCI		(2,522)	928	(4,757)	14,285
Tax effect		612	(332)	1,109	(5,778)
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI	[2]	(160)	(153,301)	(480)	(164,912)
Tax effect	[2]	42	62,276	123	66,702
Amortization of net actuarial loss	[2]	(1,205)	(1,958)	(3,615)	(15,792)
Amortization of prior service credit	[2]	1,045	1,045	3,135	3,268
Settlement costs	[2]	$ 0	$ (152,388)	$ 0	$ (152,388)

[1] The current year net realized losses related to sales of debt securities and the prior year net realized gains related to sales of debt and equity securities.

[2] These components of AOCI are components of net periodic cost. See Note 10 Employee Benefit Plans for additional details.