Other Comprehensive Income (Loss) (Component of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, before tax amount	$ 418,556	$ (182,340)	$ 157,016
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, before tax amount	(1,292)	(8,818)	(16,193)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, before tax amount	(293)	156	4,941
Total securities available for sale and transferred securities before tax amount	416,971	(191,002)	145,764
Change in the net actuarial gain/loss before tax amount	(3,644)	(7,225)	(597)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	5,623	5,002	5,429
Total defined-benefit post-retirement benefit plans	1,979	(2,223)	4,832
Other comprehensive income (loss), before tax	418,950	(193,225)	150,596
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, tax expense (benefit)	87,897	(38,292)	48,626
Securities available for sale and transferred securities, change in net unrealized gain on securities transfers to held-to-maturity, tax expense benefit	(271)	(1,853)	(5,668)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, tax expense (benefit)	(62)	33	1,729
Total securities available for sale and transferred securities, tax expense (benefit)	87,564	(40,112)	44,687
Change in the net actuarial gain/loss, tax expense (benefit)	(765)	(1,517)	(126)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit) , tax expense (benefit)	1,181	1,051	1,900
Total defined-benefit post-retirement benefit plans, tax expense (benefit)	416	(466)	1,774
Total other comprehensive income, tax expense (benefit)	87,980	(40,578)	46,461
Securities available for sale and transferred securities, change in net unrealized gain/loss during the period, net of tax amount	330,659	(144,048)	108,390
Securities available for sale and transferred securities. change in net unrealized gain on securities transfers to held-to-maturity, net of tax amount	(1,021)	(6,965)	(10,525)
Securities available for sale and securities transferred, reclassification adjustment for net (gains) losses included in net income, net of tax amount	(231)	123	3,212
Total securities available for sale and transferred securities, net of tax amount	329,407	(150,890)	101,077
Change in the net actuarial gain/loss, net of tax amount	(2,879)	(5,708)	(471)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit), net of tax amount	4,442	3,951	3,529
Total defined-benefit post-retirement benefit plans, net of tax amount	1,563	(1,757)	3,058
Other comprehensive income (loss), net of tax	$ 330,970	$ (152,647)	$ 104,135