Other Comprehensive Income (Loss) (Component of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 157,016	$ (175,061)	$ (12,450)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(16,193)	(32,207)	(33,601)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	4,941	(14,975)	(69)
Total securities available for sale and transferred securities	145,764	(222,243)	(46,120)
Change in the net actuarial gain/loss	(597)	1,914	(3,877)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit)	5,429	7,274	6,995
Total defined-benefit post-retirement benefit plans	4,832	9,188	3,118
Other comprehensive income (loss), before tax	150,596	(213,055)	(43,002)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	48,626	(61,271)	(4,358)
Securities available for sale and transferred securities, change in net unrealized gain on securities transfers to held-to-maturity, tax expense benefit	(5,668)	(11,272)	(11,760)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	1,729	(5,242)	(24)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	44,687	(77,785)	(16,142)
Change in the net actuarial gain/loss, Tax Expense (Benefit)	(126)	670	(1,357)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost benefit , Tax Expense (Benefit)	1,900	2,546	2,448
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	1,774	3,216	1,091
Total other comprehensive income, Tax Expense (Benefit)	46,461	(74,569)	(15,051)
Securities available for sale and transferred securities, change in net unrealized gain/loss during the period, net of tax	108,390	(113,790)	(8,092)
Securities available for sale and transferred securities. change in net unrealized gain on securities transfers to held-to-maturity, net of tax	(10,525)	(20,935)	(21,841)
Securities available for sale and securities transferred, reclassification adjustment for net (gains) losses included in net income, net of tax	3,212	(9,733)	(45)
Total securities available for sale and transferred securities, net of tax	101,077	(144,458)	(29,978)
Change in the net actuarial gain/loss, Net of Tax Amount	(471)	1,244	(2,520)
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	3,529	4,728	4,547
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	3,058	5,972	2,027
Other comprehensive income (loss), net of tax	$ 104,135	$ (138,486)	$ (27,951)