Retirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14,238	$ 13,372	$ 12,895
Interest cost	25,316	25,501	25,781
Expected return on plan assets	(68,575)	(63,010)	(58,504)
Amortization of prior service cost (credit)	173	313	561
Amortization of actuarial losses (gains)	7,378	7,077	8,573
Special termination benefits	0	0	2,837
Curtailment loss (gain)	0	0	183
Net periodic benefit credit	(21,470)	(16,747)	(7,674)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	99	104	222
Interest cost	640	692	891
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	(1,392)	(3,418)	(3,571)
Amortization of actuarial losses (gains)	(506)	(596)	(599)
Curtailment loss (gain)	0	0	(823)
Net periodic benefit credit	$ (1,159)	$ (3,218)	$ (3,880)