Retirement Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension Benefit Cost (Credit)
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,962	$ 2,937	$ 6,425	$ 9,507
Interest cost	8,617	5,131	26,050	15,430
Expected return on plan assets	(10,211)	(17,220)	(30,587)	(51,849)
Amortization of prior service cost	29	62	82	170
Amortization of actuarial losses (gains)	2,845	1,370	8,729	3,682
Net periodic benefit cost (credit)	3,242	(7,720)	10,699	(23,060)
Other Postretirement Benefit Credit
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	5	38	56
Interest cost	181	96	559	314
Amortization of prior service cost	(237)	(415)	(705)	(1,247)
Amortization of actuarial losses (gains)	(141)	(75)	(464)	(229)
Net periodic benefit cost (credit)	$ (185)	$ (389)	$ (572)	$ (1,106)