Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,789	$ 3,489	$ 7,577	$ 6,975
Interest cost	4,355	5,710	8,709	11,420
Expected return on plan assets	(19,850)	(17,993)	(39,698)	(35,987)
Amortization of prior service cost	58	56	116	111
Amortization of actuarial losses	3,028	2,934	6,054	5,869
Net periodic benefit credit	(8,620)	(5,804)	(17,242)	(11,612)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	25	56	48
Interest cost	95	141	190	283
Amortization of prior service cost	(457)	(484)	(913)	(967)
Amortization of actuarial losses	(47)	(69)	(95)	(139)
Net periodic benefit credit	$ (381)	$ (387)	$ (762)	$ (775)