Benefit Plans - Additional Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	1,086
Expected Benefit Payments for the Year Ending: Current plus 1	1,174
Expected Benefit Payments for the Year Ending: Current plus 2	1,368
Expected Benefit Payments for the Year Ending: Current plus 3	1,689
Expected Benefit Payments for the Year Ending: Current plus 4	1,944
Expected Benefit Payments for the Year Ending: Current plus 5	2,044
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	11,914
Effect of 1-percentage point increase on total service and interest cost component	252
Effect of 1-percentage point decrease on total service and interest cost components	(217)
Effect of 1-percentage point increase on postretirement benefit obligation	3,093
Effect of 1-percentage point decrease on postretirement benefit obligation	$ (2,748)
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	$ 9,435	$ 11,369
Current plus 1	10,950
Actual Benefit Payments for the Year Ending: Current	17,913
Expected Benefit Payments for the Year Ending: Current plus 1	28,639
Expected Benefit Payments for the Year Ending: Current plus 2	33,127
Expected Benefit Payments for the Year Ending: Current plus 3	29,669
Expected Benefit Payments for the Year Ending: Current plus 4	31,085
Expected Benefit Payments for the Year Ending: Current plus 5	31,899
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	$ 151,512