CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accumulated depreciation on fixed assets	$ 312,927	$ 290,114
Preferred stock, par value (USD per share)	$ 0.01	$ 0.01
Preferred stock authorized (shares)	15,000,000	15,000,000
Preferred stock issued (shares)	0	0
Common stock, par value (USD per share)	$ 0.01	$ 0.01
Common stock authorized (shares)	200,000,000	200,000,000
Common stock issued (shares)	95,392,547	95,437,434

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Face amount or stated value per share of common stock.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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