Postretirement Benefits - Projected and Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Projected Benefit Obligation in Excess of Plan Assets
Projected benefit obligation	$ 1,054,467	$ 886,165
Plan assets	907,625	767,569
Supplemental employee retirement plan (SERP) \| Nonqualified Plan \| Unfunded plan
Accumulated Benefit Obligation in Excess of Plan Assets
Accumulated benefit obligation	23,411	18,908
Plan assets	$ 0	$ 0

Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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