Debt and Credit Agreements - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (5,152)	$ (6,109)
Long-term debt, net	1,285,848	1,521,891
Current maturities	67,000	304,000
6.51% weighted-average senior notes
Debt Instrument [Line Items]
Total debt	$ 124,000	$ 361,000
Weighted average interest rate	6.51%	6.51%
Current maturities		$ 237,000
9.78% senior notes
Debt Instrument [Line Items]
Total debt	$ 67,000	$ 67,000
Long-term debt, interest rate, stated percentage	9.78%	9.78%
Current maturities	$ 67,000
5.58% weighted-average senior notes
Debt Instrument [Line Items]
Total debt	$ 175,000	$ 175,000
Weighted average interest rate	5.58%	5.58%
3.65% weighted-average senior notes
Debt Instrument [Line Items]
Total debt	$ 925,000	$ 925,000
Weighted average interest rate	3.65%	3.65%
Revolving credit facility
Debt Instrument [Line Items]
Total debt	$ 0	$ 0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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