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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,259	$ 5,916	$ 18,778	$ 17,748
Interest cost on projected benefit obligation	20,853	17,754	62,558	53,261
Expected return on plan assets	(27,118)	(23,527)	(81,355)	(70,578)
Amortization of prior service credit	(807)	(1,501)	(2,421)	(4,504)
Amortization of net actuarial loss	7,536	11,207	22,609	33,619
Net periodic benefit cost	6,723	9,849	20,169	29,546
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	399	188	1,197	562
Interest cost on projected benefit obligation	1,242	651	3,728	1,953
Amortization of prior service credit	(339)	(340)	(1,019)	(1,019)
Amortization of net actuarial loss	15	26	45	79
Net periodic benefit cost	$ 1,317	$ 525	$ 3,951	$ 1,575