Pension Plan and Other Postretirement Benefits (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan
Pension Plan and Other Postretirement Benefits
Employer cash contributions			$ 22,200,000	$ 20,600,000
Estimated cash contributions in the current fiscal year			29,800,000
Components of the pension plans and other postretirement benefits
Service cost	$ 6,122,000	$ 5,594,000	17,851,000	15,728,000
Interest cost	5,861,000	5,764,000	17,442,000	16,670,000
Expected return on plan assets	(6,031,000)	(5,462,000)	(18,090,000)	(16,370,000)
Amortization of prior service cost	1,445,000	1,555,000	4,336,000	4,664,000
Recognized net actuarial loss	1,881,000	1,743,000	5,386,000	4,329,000
Net periodic benefit cost	9,278,000	9,194,000	26,925,000	25,021,000
Other Benefits
Pension Plan and Other Postretirement Benefits
Employer cash contributions			2,300,000.0	6,700,000
Estimated cash contributions in the current fiscal year			9,000,000
Components of the pension plans and other postretirement benefits
Service cost	2,169,000	1,045,000	6,207,000	5,653,000
Interest cost	1,491,000	625,000	4,472,000	4,225,000
Expected return on plan assets	(1,218,000)	(1,005,000)	(3,653,000)	(3,097,000)
Amortization of prior service cost	11,000	10,000	32,000	32,000
Recognized net actuarial loss	649,000	(482,000)	1,947,000	2,041,000
Net periodic benefit cost	$ 3,102,000	$ 193,000	$ 9,005,000	$ 8,854,000