Pension and Other Postretirement Plans (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Defined Benefit Plan Deferred Pension And Other Postretirement Benefit Plan Costs Recovered [Abstract]
Deferred (recognized) pension and OPEB costs	$ (400,000)	$ 0	$ 6,200,000
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,316,000	1,141,000
Interest cost	3,023,000	3,452,000
Expected return on assets	(6,811,000)	(5,547,000)
Amortization of:
Prior service credit	(31,000)	(28,000)
Actuarial loss	1,286,000	1,729,000
Net periodic benefit (credit) cost	(1,217,000)	747,000
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	311,000	219,000
Interest cost	573,000	577,000
Expected return on assets	(790,000)	(681,000)
Amortization of:
Transition obligation	1,000	1,000
Prior service credit	(667,000)	(667,000)
Actuarial loss	70,000	106,000
Net periodic benefit (credit) cost	$ (502,000)	$ (445,000)