Employee Benefit Plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Components of Net Period Benefit Cost
Service cost	$ 2,667,000	$ 2,096,000	$ 5,334,000	$ 4,192,000
Other cost:
Interest cost	4,970,000	5,608,000	9,939,000	11,216,000
Expected return on plan assets	(6,286,000)	(5,483,000)	(12,573,000)	(10,966,000)
Recognized net actuarial loss	978,000	887,000	1,957,000	1,774,000
Total other components of the net periodic benefit cost	(338,000)	1,012,000	(677,000)	2,024,000
Net periodic benefit cost	2,329,000	3,108,000	4,657,000	6,216,000
Contribution to defined benefit and other postretirement plans	$ 0	$ 0	$ 0	$ 0