Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,130	$ 6,090	$ 15,390	$ 18,270
Interest cost on projected benefit obligation	17,290	15,032	51,871	45,097
Expected return on plan assets	(22,892)	(21,838)	(68,676)	(65,515)
Amortization of prior service cost	(1,638)	(1,639)	(4,914)	(4,917)
Amortization of net actuarial loss	3,624	10,269	10,871	30,807
Net periodic benefit cost	1,514	7,914	4,542	23,742
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	151	186	453	557
Interest cost on projected benefit obligation	695	673	2,084	2,018
Expected return on plan assets
Amortization of prior service cost	(340)	(340)	(1,019)	(1,019)
Amortization of net actuarial loss		90		270
Net periodic benefit cost	$ 506	$ 609	$ 1,518	$ 1,826