Benefit Plans - Components of Net Periodic Pension Income for Qualified and Non-Qualified Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Interest cost	$ 4,941	$ 6,113	$ 5,542
Expected return on plan assets	(17,200)	(16,453)	(15,912)
Amortization of net loss	1,003	264	625
Total net periodic pension income	$ (11,256)	$ (10,076)	$ (9,745)