Benefit Plans - Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Postretirement healthcare benefits
Assumed healthcare cost trend rates (reaches ultimate trend rate in 2026)
Healthcare cost trend rate assumed for next year (as a percent)	7.00%	7.00%	7.25%
Rate to which the cost trend is assumed to decline (the ultimate trend rate) (as a percent)	5.00%	5.00%	5.00%
Pre-tax effect of a one percentage point change in the assumed healthcare cost trend rates
Effect on 2018 total of service and interest cost components, One Percentage Point Increase	$ 77
Effect on 2018 total of service and interest cost components, One Percentage Point Decrease	(69)
Effect on postretirement benefit obligation at December 31, 2018, One Percentage Point Increase	424
Effect on postretirement benefit obligation at December 31, 2018, One Percentage Point Decrease	$ (393)
Qualified Plan (ERP) \| Pension Benefits
Weighted average assumptions were used to determine benefit obligations
Discount rate (as a percent)	4.30%	3.51%
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.51%	4.05%	4.40%
Expected long term return on plan assets (as a percent)	4.75%	4.75%	5.50%
Nonqualified Plan \| Pension Benefits
Weighted average assumptions were used to determine benefit obligations
Discount rate (as a percent)	4.30%	3.51%
Rate of compensation increase (as a percent)	4.00%	4.00%
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.51%	4.05%	4.40%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%
Nonqualified Plan \| Other Benefits
Weighted average assumptions were used to determine benefit obligations
Discount rate (as a percent)	4.30%	3.51%
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.51%	4.05%	4.40%

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

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- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

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Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate.

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

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