Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value disclosures
Long-term debt	$ 1,522,572	$ 1,521,891
Current maturities	(304,000)	(304,000)
Long-term debt, excluding current maturities	1,218,572	1,217,891
Carrying Amount
Fair value disclosures
Long-term debt	1,522,572	1,521,891
Current maturities	(304,000)	(304,000)
Long-term debt, excluding current maturities	1,218,572	1,217,891
Estimated Fair Value
Fair value disclosures
Long-term debt	1,486,210	1,527,624
Current maturities	(305,966)	(312,055)
Long-term debt, excluding current maturities	$ 1,180,244	$ 1,215,569

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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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