Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI		$ (2,215)	$ 8,839	$ (2,788)	$ 2,179
Amortization of net actuarial loss				(2,410)
Amortization of prior service credit				2,090
Unrealized Gains (Losses) on Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on sales of securities	[1]	(2,055)	14,729	(2,468)	13,790
Reclassifications out of AOCI		(2,055)	14,729	(2,468)	13,790
Tax effect		467	(5,244)	570	(5,274)
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications out of AOCI	[2]	(160)	(5,890)	(320)	(11,611)
Tax effect	[2]	41	2,253	81	4,441
Amortization of net actuarial loss	[2]	(1,205)	(6,902)	(2,410)	(13,834)
Amortization of prior service credit	[2]	$ 1,045	$ 1,012	$ 2,090	$ 2,223

[1] For the three and six months ended June 30, 2018, net realized losses of $2.1 million and $2.5 million, respectively, related to sales of debt securities classified as available-for-sale. For the three and six months ended June 30, 2017, net realized gains of $14.7 million and $13.8 million, respectively, related to sales of debt and equity securities classified as available-for-sale.

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