Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,324	$ 5,086	$ 12,970	$ 15,258
Interest cost on projected benefit obligation	20,394	18,676	61,184	56,029
Expected return on plan assets	(30,534)	(30,781)	(91,604)	(92,344)
Amortization of prior service cost (credit)	139	139	418	417
Amortization of net actuarial loss (gain)	5,498	10,948	16,494	32,844
Net periodic cost (benefit)	(179)	4,068	(538)	12,204
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	215	234	644	703
Interest cost on projected benefit obligation	586	573	1,758	1,719
Amortization of prior service cost (credit)	(1,182)	(1,182)	(3,547)	(3,546)
Amortization of net actuarial loss (gain)	(312)	(206)	(935)	(619)
Net periodic cost (benefit)	$ (693)	$ (581)	$ (2,080)	$ (1,743)