Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,177	$ 2,983	$ 10,754	$ 9,958
Interest cost	4,579	5,816	13,288	17,236
Expected return on plan assets	(19,892)	(18,104)	(59,590)	(54,091)
Amortization of prior service cost	67	43	183	154
Amortization of actuarial losses	3,464	2,993	9,518	8,862
Net periodic benefit credit	(8,605)	(6,269)	(25,847)	(17,881)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	25	18	81	66
Interest cost	82	142	272	425
Amortization of prior service cost	(460)	(487)	(1,373)	(1,454)
Amortization of actuarial losses	(137)	(115)	(232)	(254)
Net periodic benefit credit	$ (490)	$ (442)	$ (1,252)	$ (1,217)