Retirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,688	$ 3,392	$ 11,689	$ 10,163
Interest cost	6,753	7,399	21,044	22,248
Expected return on plan assets	(15,692)	(14,312)	(47,045)	(43,000)
Amortization of prior service cost	264	272	778	891
Amortization of actuarial losses (gains)	627	154	4,293	588
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments	(482)		(482)
Net periodic benefit credit	(4,842)	(3,095)	(9,723)	(9,110)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	56	138	335	402
Interest cost	256	420	965	1,245
Amortization of prior service cost	(817)	(726)	(2,289)	(2,154)
Amortization of actuarial losses (gains)	(133)	(75)	(260)	(246)
Net periodic benefit credit	$ (638)	$ (243)	$ (1,249)	$ (753)