Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Costs of Pension and Post Retirement Medical 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	$ 2,614	$ 2,850	$ 7,855	$ 8,546
Interest cost	6,120	5,868	18,360	17,606
Expected return on plan assets	(11,331)	(11,498)	(33,992)	(34,495)
Amortization of prior service cost	674	760	2,023	2,281
Amortization of unrecognized (gain)/loss	3,293	4,226	9,880	12,679
Net periodic benefit cost	1,370	2,206	4,126	6,617
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	287	358	860	1,074
Interest cost	498	499	1,494	1,498
Amortization of prior service cost	(45)	(70)	(136)	(210)
Amortization of unrecognized (gain)/loss	(189)	58	(567)	173
Net periodic benefit cost	$ 551	$ 845	$ 1,651	$ 2,535