Benefit Plans and Other Postretirement Benefits, Net pension expense (Details) - Pension Benefits - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Components of net pension expense:
Service cost	$ 1,800,000	$ 1,700,000	$ 3,800,000	$ 3,500,000
Interest cost	4,100,000	5,400,000	8,300,000	10,800,000
Expected return on plan assets	(9,300,000)	(9,200,000)	(18,600,000)	(18,500,000)
Amortization of prior service cost	600,000	400,000	1,100,000	800,000
Amortization of net actuarial losses	6,300,000	4,600,000	12,500,000	9,300,000
Net pension expense	3,500,000	$ 2,900,000	7,100,000	$ 5,900,000
United States
Defined Benefit Plan Disclosure
Estimated future employer contribution in fiscal year	$ 0		$ 0