Other Comprehensive Income (Loss) (Component of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, before tax amount	$ (182,340)	$ 157,016	$ (175,061)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, before tax amount	(8,818)	(16,193)	(32,207)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, before tax amount	156	4,941	(14,975)
Total securities available for sale and transferred securities before tax amount	(191,002)	145,764	(222,243)
Change in the net actuarial gain/loss before tax amount	(7,225)	(597)	1,914
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	5,002	5,429	7,274
Total defined-benefit post-retirement benefit plans	(2,223)	4,832	9,188
Other comprehensive income (loss), before tax	(193,225)	150,596	(213,055)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, tax expense (benefit)	(38,292)	48,626	(61,271)
Securities available for sale and transferred securities, change in net unrealized gain on securities transfers to held-to-maturity, tax expense benefit	(1,853)	(5,668)	(11,272)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, tax expense (benefit)	33	1,729	(5,242)
Total securities available for sale and transferred securities, tax expense (benefit)	(40,112)	44,687	(77,785)
Change in the net actuarial gain/loss, tax expense (benefit)	(1,517)	(126)	670
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit) , tax expense (benefit)	1,051	1,900	2,546
Total defined-benefit post-retirement benefit plans, tax expense (benefit)	(466)	1,774	3,216
Total other comprehensive income, tax expense (benefit)	(40,578)	46,461	(74,569)
Securities available for sale and transferred securities, change in net unrealized gain/loss during the period, net of tax amount	(144,048)	108,390	(113,790)
Securities available for sale and transferred securities. change in net unrealized gain on securities transfers to held-to-maturity, net of tax amount	(6,965)	(10,525)	(20,935)
Securities available for sale and securities transferred, reclassification adjustment for net (gains) losses included in net income, net of tax amount	123	3,212	(9,733)
Total securities available for sale and transferred securities, net of tax amount	(150,890)	101,077	(144,458)
Change in the net actuarial gain/loss, net of tax amount	(5,708)	(471)	1,244
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	3,951	3,529	4,728
Total defined-benefit post-retirement benefit plans, net of tax amount	(1,757)	3,058	5,972
Other comprehensive income (loss), net of tax	$ (152,647)	$ 104,135	$ (138,486)