Long-Term Debt - By Issue (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Long Term Debt
Total debt	$ 2,408,852		$ 2,386,791
Less debt issuance costs	26,912		29,965
Total amounts outstanding	2,381,940		2,356,826
Less current maturities	28,795		3,632
Long-term debt	2,353,145		2,353,194
5.125% senior notes due 2021
Long Term Debt
Total debt	700,000		700,000
Total amounts outstanding	$ 700,000
Stated interest rate	5.125%
5 1/4% Senior Notes due 2023
Long Term Debt
Total debt	$ 400,000		400,000
Total amounts outstanding	$ 400,000
Stated interest rate	5.25%
5.500% senior notes due 2024
Long Term Debt
Total debt	$ 500,000		500,000
Total amounts outstanding	$ 500,000
Stated interest rate	5.50%
4.125% Senior Note due 2025 [Member]
Long Term Debt
Total debt	$ 350,000
Total amounts outstanding	$ 350,000
Stated interest rate	4.125%	4.125%
Senior Notes 5.000 Percent Due 2026 [Member]
Long Term Debt
Total debt	$ 400,000		400,000
Total amounts outstanding	$ 400,000
Stated interest rate	5.00%
6 3/8% Senior Notes, due 2022
Long Term Debt
Total debt			350,000
Stated interest rate	6.375%
6 1/8% senior notes, due 2019
Long Term Debt
Stated interest rate	6.125%
Other obligations
Long Term Debt
Total debt	$ 58,852		$ 36,791