Benefit Plans - Additional Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Effect of 1-percentage point increase on total service and interest cost component	$ 237
Effect of 1-percentage point decrease on total service and interest cost components	(205)
Effect of 1-percentage point increase on postretirement benefit obligation	2,382
Effect of 1-percentage point decrease on postretirement benefit obligation	(2,102)
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	1,355
Expected Benefit Payments for the Year Ending: Current plus 1	847
Expected Benefit Payments for the Year Ending: Current plus 2	978
Expected Benefit Payments for the Year Ending: Current plus 3	1,068
Expected Benefit Payments for the Year Ending: Current plus 4	1,257
Expected Benefit Payments for the Year Ending: Current plus 5	1,410
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	$ 8,574
Other Postretirement Benefit Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations, minimum (in hundredths)	70.00%
Target asset allocations, maximum (in hundredths)	100.00%
Other Postretirement Benefit Plan | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations, minimum (in hundredths)	0.00%
Target asset allocations, maximum (in hundredths)	15.00%
Other Postretirement Benefit Plan | Commodities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations, minimum (in hundredths)	0.00%
Target asset allocations, maximum (in hundredths)	10.00%
Other Postretirement Benefit Plan | Fixed Income/Cash
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations, minimum (in hundredths)	0.00%
Target asset allocations, maximum (in hundredths)	10.00%
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Company Contributions	$ 11,369	$ 17,311
Current plus 1	9,500
Actual Benefit Payments for the Year Ending: Current	24,500
Expected Benefit Payments for the Year Ending: Current plus 1	21,831
Expected Benefit Payments for the Year Ending: Current plus 2	23,439
Expected Benefit Payments for the Year Ending: Current plus 3	26,927
Expected Benefit Payments for the Year Ending: Current plus 4	27,199
Expected Benefit Payments for the Year Ending: Current plus 5	27,151
Expected Benefit Payments for the Year Ending: Current plus 6 - 10	$ 146,471