Benefit Plans - Additional Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Company Contributions	$ 0	$ 0
Current plus 1	0
Actual Benefit Payments for the Year Ending: Current	652
Expected Benefit Payments for the Year Ending: Current plus 1	1,352
Expected Benefit Payments for the Year Ending: Current plus 2	1,650
Expected Benefit Payments for the Year Ending: Current plus 3	1,916
Expected Benefit Payments for the Year Ending: Current plus 4	2,386
Expected Benefit Payments for the Year Ending: Current plus 5	2,613
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	14,065
Effect of 1-percentage point increase on total service and interest cost component	327
Effect of 1-percentage point decrease on total service and interest cost components	(282)
Effect of 1-percentage point increase on postretirement benefit obligation	3,221
Effect of 1-percentage point decrease on postretirement benefit obligation	$ (2,866)
Other Postretirement Benefit Plan | Equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	70.00%
Other Postretirement Benefit Plan | Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	100.00%
Other Postretirement Benefit Plan | Real Estate | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	0.00%
Other Postretirement Benefit Plan | Real Estate | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	15.00%
Other Postretirement Benefit Plan | Commodities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	0.00%
Other Postretirement Benefit Plan | Commodities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	10.00%
Other Postretirement Benefit Plan | Fixed Income/Cash | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	0.00%
Other Postretirement Benefit Plan | Fixed Income/Cash | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations (in hundredths)	10.00%
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Contributions	$ 10,908	$ 9,435
Current plus 1	10,650
Actual Benefit Payments for the Year Ending: Current	33,582
Expected Benefit Payments for the Year Ending: Current plus 1	33,258
Expected Benefit Payments for the Year Ending: Current plus 2	28,688
Expected Benefit Payments for the Year Ending: Current plus 3	30,574
Expected Benefit Payments for the Year Ending: Current plus 4	30,490
Expected Benefit Payments for the Year Ending: Current plus 5	30,510
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	$ 143,389