Postretirement Benefits (Details 6) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair value of plan assets:
Fair value of plan assets, beginning of year	$ 592,544	$ 551,491
Actual gain on plan assets	108,618	38,028
Employer contributions	61,102	17,418
Benefits paid	(16,184)	(14,393)
Settlements	(2,180)
Fair value of plan assets, end of year	$ 743,900	$ 592,544
Supplemental employee retirement plan (SERP) \| Nonqualified plan \| Unfunded plan
Fair value of plan assets:
Number of former plan participants resulting in charges recognized as settlements	2

The number of former plan participants for which settlement charges were recognized.

No definition available.

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- Definition

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

+ References

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

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- Definition

Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

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- Definition

Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract.

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