Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,489	$ 3,254	$ 6,975	$ 6,512
Interest cost	5,710	7,043	11,420	14,092
Expected return on plan assets	(17,993)	(15,112)	(35,987)	(30,225)
Amortization of prior service cost (credit)	56	21	111	40
Amortization of actuarial losses (gains)	2,934	4,121	5,869	8,240
Net periodic benefit credit	(5,804)	(673)	(11,612)	(1,341)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	25	22	48	44
Interest cost	141	184	283	369
Amortization of prior service cost (credit)	(484)	(581)	(967)	(1,163)
Amortization of actuarial losses (gains)	(69)	(166)	(139)	(333)
Net periodic benefit credit	$ (387)	$ (541)	$ (775)	$ (1,083)