Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on securities						$ 7,480
Income tax expense		$ (33,145)		$ (36,451)		(64,133)	$ (74,376)
Net income		$ 97,246	$ 97,577	$ 107,354	$ 106,552	194,823	$ 213,906
Reclassifications, net of tax	[1]					352
Past service liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1],[2]					124
Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1],[2]					(5,080)
Amortization of defined benefit pension
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1]					(4,956)
Tax benefit	[1]					1,390
Reclassifications, net of tax	[1]					(3,566)
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 (loss) on securities	[1]					5,445
Income tax expense	[1]					(1,527)
Net income	[1]					$ 3,918

[1] Amounts in parentheses indicate expense items.

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