Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions	$ 7,300,000	$ 7,500,000
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
Interest cost	5,995,000	5,791,000
Expected return on plan assets	(6,862,000)	(6,029,000)
Amortization of prior service cost	1,263,000	1,445,000
Recognized net actuarial loss	2,797,000	1,752,000
Net periodic benefit cost	10,595,000	8,824,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions	0	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
Interest cost	1,484,000	1,491,000
Expected return on plan assets	(1,416,000)	(1,218,000)
Amortization of prior service cost	11,000	11,000
Recognized net actuarial loss	773,000	649,000
Net periodic benefit cost	$ 3,402,000	$ 2,952,000