Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 6,300,000	$ 16,300,000
Estimated cash contributions in the current fiscal year	$ 18,800,000		18,800,000
Components of the pension plans and other postretirement benefits
Service cost	6,565,000	$ 7,402,000	13,129,000	14,804,000
Interest cost	6,642,000	5,995,000	13,284,000	11,989,000
Expected return on plan assets	(7,567,000)	(6,862,000)	(15,133,000)	(13,725,000)
Amortization of prior service cost	1,262,000	1,263,000	2,524,000	2,526,000
Recognized net actuarial loss	1,312,000	2,797,000	2,624,000	5,595,000
Net periodic benefit cost	8,214,000	10,595,000	16,428,000	21,189,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			3,400,000	2,700,000
Estimated cash contributions in the current fiscal year	7,900,000		7,900,000
Components of the pension plans and other postretirement benefits
Service cost	1,762,000	2,550,000	3,524,000	5,100,000
Interest cost	1,337,000	1,484,000	2,674,000	2,969,000
Expected return on plan assets	(1,435,000)	(1,416,000)	(2,871,000)	(2,832,000)
Amortization of prior service cost	49,000	11,000	99,000	21,000
Recognized net actuarial loss	104,000	773,000	207,000	1,547,000
Net periodic benefit cost	$ 1,817,000	$ 3,402,000	$ 3,633,000	$ 6,805,000