Retirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12,505	$ 14,238	$ 13,372
Interest cost	28,316	25,316	25,501
Expected return on plan assets	(60,567)	(68,575)	(63,010)
Amortization of prior service cost (credit)	115	173	313
Amortization of actuarial losses (gains)	16,399	7,378	7,077
Net periodic benefit credit	(3,232)	(21,470)	(16,747)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	99	104
Interest cost	759	640	692
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	(2,330)	(1,392)	(3,418)
Amortization of actuarial losses (gains)	(488)	(506)	(596)
Net periodic benefit credit	$ (1,979)	$ (1,159)	$ (3,218)