Pension Plans - Components of Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Retirement Benefits [Abstract]
Service cost	$ 6,748	$ 7,545	$ 13,535	$ 15,081
Interest cost	1,241	1,246	2,483	2,493
Expected return on plan assets	(1,434)	(1,381)	(2,863)	(2,755)
Recognized actuarial loss	33	14	64	28
Net periodic pension cost	$ 6,588	$ 7,424	$ 13,219	$ 14,847