Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 1,346	$ 1,708
Interest cost	4,343	3,867
Expected return on plan assets	(6,100)	(6,185)
Prior service cost (benefit)	6	15
Net loss (gain)	941	2,158
Net periodic benefit cost	536	1,563
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer	0
Estimated Future Employer Contributions in Current Fiscal Year	12,700
Estimated Future Employer Contributions in Next Fiscal Year	12,700
Defined Benefit Postretirement Healthcare Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	454	573
Interest cost	560	521
Expected return on plan assets	(57)	(57)
Prior service cost (benefit)	(99)	(99)
Net loss (gain)	0	54
Net periodic benefit cost	858	992
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer	1,109
Estimated Future Employer Contributions in Current Fiscal Year	3,326
Estimated Future Employer Contributions in Next Fiscal Year	4,271
Supplemental Non-qualified Defined Benefit and Defined Contribution Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1,285	280
Interest cost	324	293
Net loss (gain)	134	250
Net periodic benefit cost	1,743	$ 823
Payment for Pension and Other Postretirement Benefits [Abstract]
Contributions by Employer	366
Estimated Future Employer Contributions in Current Fiscal Year	1,097
Estimated Future Employer Contributions in Next Fiscal Year	$ 1,562