Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (loss) gain on securities		$ (303)	$ 26,936	$ (769)	$ 28,915
Income tax expense		(36,451)	(65,447)	(74,376)	(125,644)
Net (Loss) income		$ 107,354	$ 115,255	213,906	$ 219,212
Reclassifications, net of tax	[1]			(2,555)
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]			(3,744)
Amortization of defined benefit pension
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1]			(3,620)
Tax benefit	[1]			1,065
Reclassifications, net of tax	[1]			(2,555)
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 (loss) gain on securities	[1]			0
Income tax expense	[1]			0
Net (Loss) income	[1]			$ 0

[1] Amounts in parentheses indicate expense items.

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