Retirement Benefits - Narrative (Details) - Defined Benefit Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net periodic pension expense for defined benefit pension plans
Service cost	$ 4,375	$ 3,948	$ 8,759	$ 7,929
Interest cost	2,388	4,924	4,782	9,899
Expected return on assets	(6,340)	(8,193)	(12,696)	(16,472)
Amortization of prior service credit	(218)	(222)	(436)	(447)
Recognized net actuarial loss	1,399	2,588	2,801	5,207
Net periodic pension expense	1,604	$ 3,045	3,210	$ 6,116
Company contributions			10,000
Maximum
Net periodic pension expense for defined benefit pension plans
Expected contributions during 2018	$ 15,000		$ 15,000