Retirement Benefits - Narrative (Details) - Defined Benefit Pension Plans - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net periodic pension expense for defined benefit pension plans
Service cost	$ 3,917	$ 4,436	$ 11,846	$ 13,644
Interest cost	4,832	5,537	14,731	17,118
Expected return on assets	(8,068)	(9,685)	(24,540)	(29,919)
Amortization of prior service credit	(220)	(230)	(667)	(709)
Recognized net actuarial loss	2,529	1,803	7,736	5,573
Loss on settlement	0	18,641	0	20,534
Net periodic pension expense	2,990	$ 20,502	9,106	$ 26,241
Company contributions			11,000
Maximum
Net periodic pension expense for defined benefit pension plans
Expected contributions during 2018	$ 15,000		$ 15,000