Long-Term Debt and Notes Payable - Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Gross amount	$ 750,500	$ 1,039,000
Less unamortized debt discount	419	437
Long-term debt	750,081	1,038,563
Credit facility
Debt Instrument [Line Items]
Gross amount	196,500	485,000
Medium-term Notes | 7.40% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Gross amount	$ 2,000	$ 2,000
Stated interest rate	7.40%	7.40%
Medium-term Notes | 7.36% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Gross amount	$ 15,000	$ 15,000
Stated interest rate	7.36%	7.36%
Medium-term Notes | 7.23% Medium-term Notes, Series A, due July 28, 2017
Debt Instrument [Line Items]
Gross amount	$ 2,000	$ 2,000
Stated interest rate	7.23%	7.23%
Medium-term Notes | 7.32% Medium-term Notes, Series A, due July 28, 2022
Debt Instrument [Line Items]
Gross amount	$ 20,000	$ 20,000
Stated interest rate	7.32%	7.32%
Medium-term Notes | 7.60% Medium-term Notes, Series A, due July 26, 2027
Debt Instrument [Line Items]
Gross amount	$ 5,000	$ 5,000
Stated interest rate	7.60%	7.60%
Medium-term Notes | 7.35% Medium-term Notes, Series A, due July 28, 2027
Debt Instrument [Line Items]
Gross amount	$ 10,000	$ 10,000
Stated interest rate	7.35%	7.35%
Medium-term Notes | 7.125% Medium-term Notes, Series B, due February 15, 2028
Debt Instrument [Line Items]
Gross amount	$ 100,000	$ 100,000
Stated interest rate	7.125%	7.125%
Notes Payable | 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Gross amount	$ 400,000	$ 400,000
Stated interest rate	4.625%	4.625%