Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 33,811	$ 122,218
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(6,286)	(8,166)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(14,903)
Total securities available for sale and transferred securities, before tax amount	27,525	99,149
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, before tax amount	1,357	1,553
Total defined-benefit post-retirement benefit plans	1,357	1,553
Total other comprehensive income (loss), before tax	28,882	100,702
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	11,834	42,776
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(2,200)	(2,858)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(5,216)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	9,634	34,702
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, Tax Expense (Benefit)	475	544
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	475	544
Total other comprehensive income (loss), Tax Expense (Benefit)	10,109	35,246
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	21,977	79,442
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(4,086)	(5,308)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(9,687)
Total securities available for sale and transferred securities, Net of Tax	17,891	64,447
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income, Net of Tax	882	1,009
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	882	1,009
Other comprehensive income (loss), net of tax	$ 18,773	$ 65,456