Pension Plans - Components of Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Retirement Benefits [Abstract]
Service cost	$ 7,545	$ 7,789	$ 15,081	$ 15,766
Interest cost	1,246	1,305	2,493	2,629
Expected return on plan assets	(1,381)	(1,595)	(2,755)	(3,213)
Recognized actuarial gain	14	(94)	28	(188)
Net periodic pension cost	$ 7,424	$ 7,405	$ 14,847	$ 14,994