Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on securities		$ 534	$ 5,409
Interest expense		196,575	204,849
Income tax expense		(14,915)	(30,988)
Net income		100,328	$ 97,577
Reclassifications, net of tax	[1]	(278)
Past service liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1],[2]	62
Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1],[2]	(1,837)
Amortization of defined benefit pension
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1]	(1,775)
Tax benefit	[1]	487
Reclassifications, net of tax	[1]	(1,288)
Reclassification out of Accumulated Other Comprehensive Income (loss) | Unrealized gains on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on securities	[1]	749
Income tax expense	[1]	(206)
Net income	[1]	543
Reclassification out of Accumulated Other Comprehensive Income (loss) | Unrealized gains (loss) on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	[1]	645
Income tax expense	[1]	(178)
Net income	[1]	$ 467

[1] Amounts in parentheses indicate expense items.

[2] See Note 8, “Pension and Other Post-Retirement Benefits,” for additional information.