Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 16,000,000	$ 14,800,000
Estimated cash contributions in the current fiscal year			33,400,000
Components of the pension plans and other postretirement benefits
Service cost	$ 7,402,000	$ 5,865,000	14,804,000	11,730,000
Interest cost	5,995,000	5,791,000	11,989,000	11,581,000
Expected return on plan assets	(6,862,000)	(6,029,000)	(13,725,000)	(12,058,000)
Amortization of prior service cost	1,263,000	1,445,000	2,526,000	2,890,000
Recognized net actuarial loss	2,797,000	1,752,000	5,595,000	3,504,000
Net periodic benefit cost	10,595,000	8,824,000	21,189,000	17,647,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			2,700,000	0
Estimated cash contributions in the current fiscal year			10,100,000
Components of the pension plans and other postretirement benefits
Service cost	2,550,000	2,019,000	5,100,000	4,038,000
Interest cost	1,484,000	1,491,000	2,969,000	2,982,000
Expected return on plan assets	(1,416,000)	(1,218,000)	(2,832,000)	(2,436,000)
Amortization of prior service cost	11,000	11,000	21,000	22,000
Recognized net actuarial loss	773,000	649,000	1,547,000	1,298,000
Net periodic benefit cost	$ 3,402,000	$ 2,952,000	$ 6,805,000	$ 5,904,000