Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 1,353	$ 1,345	$ 2,706	$ 2,691
Interest cost	3,356	4,344	6,713	8,687
Expected return on plan assets	(5,648)	(6,100)	(11,296)	(12,200)
Prior service cost (benefit)	0	7	0	13
Net loss (gain)	2,093	940	4,186	1,881
Net periodic benefit cost	1,154	536	2,309	1,072
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer			12,700
Estimated Future Employer Contributions in Current Fiscal Year	0		0
Estimated Future Employer Contributions in Next Fiscal Year	12,700		12,700
Other Postretirement Benefits Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	514	454	1,028	908
Interest cost	413	563	825	1,123
Expected return on plan assets	(46)	(58)	(91)	(115)
Prior service cost (benefit)	(137)	(100)	(274)	(199)
Net loss (gain)	5	0	10	0
Net periodic benefit cost	749	859	1,498	1,717
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer			2,670
Estimated Future Employer Contributions in Current Fiscal Year	2,671		2,671
Estimated Future Employer Contributions in Next Fiscal Year	5,364		5,364
Supplemental Non-qualified Defined Benefit and Defined Contribution Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1,817	692	447	1,977
Interest cost	275	324	550	648
Prior service cost (benefit)	0	1	0	1
Net loss (gain)	426	134	852	268
Net periodic benefit cost	2,518	$ 1,151	1,849	$ 2,894
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer			710
Estimated Future Employer Contributions in Current Fiscal Year	710		710
Estimated Future Employer Contributions in Next Fiscal Year	$ 1,614		$ 1,614