Debt and Credit Agreements - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (6,109)	$ (7,470)
Long-term debt	1,521,891	1,520,530
Current portion of long-term debt	$ 304,000	$ 0
6.51% weighted-average senior notes
Debt Instrument [Line Items]
Weighted-average interest rate (as a percent)	6.51%	6.51%
Long-term debt including unamortized debt issuance costs	$ 361,000	$ 361,000
Current portion of long-term debt	$ 237,000
9.78% senior notes
Debt Instrument [Line Items]
Interest rate (as a percent)	9.78%	9.78%
Long-term debt including unamortized debt issuance costs	$ 67,000	$ 67,000
Current portion of long-term debt	$ 67,000
5.58% weighted-average senior notes
Debt Instrument [Line Items]
Weighted-average interest rate (as a percent)	5.58%	5.58%
Long-term debt including unamortized debt issuance costs	$ 175,000	$ 175,000
3.65% weighted-average senior notes
Debt Instrument [Line Items]
Weighted-average interest rate (as a percent)	3.65%	3.65%
Long-term debt including unamortized debt issuance costs	$ 925,000	$ 925,000
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 0	$ 0