Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 42,300,000	$ 22,200,000
Estimated cash contributions in the current fiscal year			42,300,000
Components of the pension plans and other postretirement benefits
Service cost	$ 6,966,000	$ 6,122,000	21,770,000	17,851,000
Interest cost	6,007,000	5,861,000	17,996,000	17,442,000
Expected return on plan assets	(7,052,000)	(6,031,000)	(20,777,000)	(18,090,000)
Amortization of prior service cost	1,263,000	1,445,000	3,789,000	4,336,000
Recognized net actuarial loss	2,791,000	1,881,000	8,386,000	5,386,000
Net periodic benefit cost	9,975,000	9,278,000	31,164,000	26,925,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			8,000,000	2,300,000.0
Estimated cash contributions in the current fiscal year			9,700,000
Components of the pension plans and other postretirement benefits
Service cost	1,966,000	2,169,000	7,066,000	6,207,000
Interest cost	1,183,000	1,491,000	4,152,000	4,472,000
Expected return on plan assets	(1,397,000)	(1,218,000)	(4,229,000)	(3,653,000)
Amortization of prior service cost	11,000	11,000	32,000	32,000
Recognized net actuarial loss	242,000	649,000	1,789,000	1,947,000
Net periodic benefit cost	$ 2,005,000	$ 3,102,000	$ 8,810,000	$ 9,005,000