Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,983	$ 2,871	$ 9,958	$ 9,383
Interest cost	5,816	7,149	17,236	21,241
Expected return on plan assets	(18,104)	(15,200)	(54,091)	(45,425)
Amortization of prior service cost	43	46	154	86
Amortization of actuarial losses	2,993	4,084	8,862	12,324
Net periodic benefit credit	(6,269)	(1,050)	(17,881)	(2,391)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	18	16	66	60
Interest cost	142	200	425	569
Amortization of prior service cost	(487)	(586)	(1,454)	(1,749)
Amortization of actuarial losses	(115)	(33)	(254)	(366)
Net periodic benefit credit	$ (442)	$ (403)	$ (1,217)	$ (1,486)