Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,788	$ 3,486
Interest cost	4,354	5,710
Expected return on plan assets	(19,848)	(17,993)
Amortization of prior service cost	58	55
Amortization of actuarial losses	3,026	2,934
Net periodic benefit credit	(8,622)	(5,808)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	23
Interest cost	95	142
Amortization of prior service cost	(456)	(483)
Amortization of actuarial losses	(48)	(70)
Net periodic benefit credit	$ (381)	$ (388)