Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions	$ 7,900,000	$ 0
Estimated cash contributions in the current fiscal year	38,000,000.0
Components of the pension plans and other postretirement benefits
Service cost	8,811,000	6,565,000
Interest cost	6,433,000	6,642,000
Expected return on plan assets	(8,265,000)	(7,567,000)
Amortization of prior service cost	1,057,000	1,262,000
Recognized net actuarial loss	3,196,000	1,312,000
Net periodic benefit cost	11,232,000	8,214,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions	2,200,000	1,400,000
Estimated cash contributions in the current fiscal year	7,500,000
Components of the pension plans and other postretirement benefits
Service cost	2,106,000	1,762,000
Interest cost	1,210,000	1,337,000
Expected return on plan assets	(1,811,000)	(1,435,000)
Amortization of prior service cost	49,000	49,000
Recognized net actuarial loss	14,000	104,000
Net periodic benefit cost	$ 1,568,000	$ 1,817,000