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Subsequent events Termination of West Virginia postemployment benefits (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	45 Months Ended
	Jan. 31, 2010	Dec. 31, 2016
Subsequent Event [Line Items]
Pension Contributions		$ 5,900
Age Which Retiree Medical Benefits Will Cease for Retirees and Dependents	65 years
Pension Expense		11,500
CAWV Total Pension Contribution over Agreement [Member]
Subsequent Event [Line Items]
Pension Contributions		$ 17,400

The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period.

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