Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,916	$ 5,130	$ 17,748	$ 15,390
Interest cost on projected benefit obligation	17,754	17,290	53,261	51,871
Expected return on plan assets	(23,527)	(22,892)	(70,578)	(68,676)
Amortization of prior service credit	(1,501)	(1,638)	(4,504)	(4,914)
Amortization of net actuarial loss	11,207	3,624	33,619	10,871
Net periodic benefit cost	9,849	1,514	29,546	4,542
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	188	151	562	453
Interest cost on projected benefit obligation	651	695	1,953	2,084
Amortization of prior service credit	(340)	(340)	(1,019)	(1,019)
Amortization of net actuarial loss	26		79
Net periodic benefit cost	$ 525	$ 506	$ 1,575	$ 1,518