Debt and Credit Agreements - Schedule of debt (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 1,528,000	$ 2,025,000
Unamortized debt issuance costs	(7,470)	(8,861)
Long-term debt	1,520,530	2,016,139
Current maturities	$ 0	$ (20,000)
7.33% weighted-average senior notes
Debt Instrument [Line Items]
Weighted-average interest rate (as a percent)	7.33%	7.33%
Long-term debt including unamortized debt issuance costs	$ 0	$ 20,000
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	$ 361,000	$ 425,000
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
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	$ 413,000