Debt and Credit Agreements - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (4,896)	$ (6,109)
Long-term debt	1,226,104	1,521,891
Current portion of long-term debt	$ 0	$ 304,000
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	$ 124,000	$ 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	$ 0	$ 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	$ 7,000	$ 0