Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss				$ (1,830)
Amortization of prior service credit				2,035
Reclassifications out of AOCI		$ (144)	$ (2,215)	(82)	$ (2,788)
Unrealized gains (losses) on debt securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of debt securities		(246)	(2,055)	(287)	(2,468)
Reclassifications out of AOCI		(246)	(2,055)	(287)	(2,468)
Tax effect		58	467	68	570
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(915)	(1,205)	(1,830)	(2,410)
Amortization of prior service credit	[1]	1,017	1,045	2,035	2,090
Reclassifications out of AOCI	[1]	102	(160)	205	(320)
Tax effect	[1]	$ (27)	$ 41	$ (54)	$ 81

[1] These components of AOCI are components of net periodic cost.  See Note 11 Employee Benefit Plans for additional details.