Debt and Credit Agreements - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 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	(7,810)	(8,861)
Debt, net	1,520,190	2,016,139
Current portion of long-term debt	0	20,000
7.33% weighted-average senior notes
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 0	$ 20,000
Weighted average interest rate	7.33%	7.33%
6.51% weighted-average senior notes
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 361,000	$ 425,000
Weighted average interest rate	6.51%	6.51%
9.78% senior notes
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 67,000	$ 67,000
Long-term debt, interest rate, stated percentage	9.78%	9.78%
5.58% weighted-average senior notes
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 175,000	$ 175,000
Weighted average interest rate	5.58%	5.58%
3.65% weighted-average senior notes
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 925,000	$ 925,000
Weighted average interest rate	3.65%	3.65%
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt including unamortized debt issuance costs	$ 0	$ 413,000