Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 7,082	$ (95,641)	$ 131,283	$ 191,865
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(3,514)	(7,278)	(13,660)	(24,629)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	4,867	37	4,917	(14,866)
Total securities available for sale and transferred securities, before tax amount	8,435	(102,882)	122,540	152,370
Change in the net actuarial gain/loss,Before Tax Amount	0	0	0	(862)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, before tax amount	1,357	1,585	4,072	4,878
Total defined-benefit post-retirement benefit plans	1,357	1,585	4,072	4,016
Total other comprehensive income (loss), before tax	9,792	(101,297)	126,612	156,386
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	2,479	(33,473)	45,949	67,154
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(1,230)	(2,547)	(4,781)	(8,620)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	1,703	12	1,721	(5,204)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	2,952	(36,008)	42,889	53,330
Change in the net actuarial gain/loss,Tax Expense (Benefit)	0	0	0	(302)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, Tax Expense (Benefit)	475	555	1,425	1,708
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	475	555	1,425	1,406
Total other comprehensive income (loss), Tax Expense (Benefit)	3,427	(35,453)	44,314	54,736
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	4,603	(62,168)	85,334	124,711
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(2,284)	(4,731)	(8,879)	(16,009)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	3,164	25	3,196	(9,662)
Total securities available for sale and transferred securities, Net of Tax	5,483	(66,874)	79,651	99,040
Change in the net actuarial gain/loss, Net of Tax	0	0	0	(560)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income, Net of Tax	882	1,030	2,647	3,170
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	882	1,030	2,647	2,610
Other comprehensive income (loss), net of tax	$ 6,365	$ (65,844)	$ 82,298	$ 101,650