Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 122,218	$ 34,527
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(8,166)	(7,887)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(14,903)	(228)
Total securities available for sale and transferred securities, before tax amount	99,149	26,412
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Before Tax Amount	1,553	1,749
Total other comprehensive income (loss), before tax	100,702	28,161
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	42,776	12,084
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(2,858)	(2,760)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(5,216)	(80)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	34,702	9,244
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Tax Expense (Benefit)	544	612
Total other comprehensive income (loss), Tax Expense (Benefit)	35,246	9,856
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	79,442	22,443
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(5,308)	(5,127)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(9,687)	(148)
Total securities available for sale and transferred securities, Net of Tax	64,447	17,168
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/lloss, Net of Tax	1,009	1,137
Other comprehensive income (loss), net of tax	$ 65,456	$ 18,305