Financing Arrangements (Narrative) (Details) - USD ($)		1 Months Ended	12 Months Ended
	Mar. 31, 2017	Aug. 31, 2016	Dec. 31, 2016	Sep. 30, 2017	Dec. 21, 2016	Sep. 30, 2016
Line of Credit Facility [Line Items]
Issue cost of debt facility			$ 14,085,000
Capital lease payment duration			15 years
Capital lease expiration date			Mar. 01, 2029
Capital lease obligations, current			$ 20,617,000
Capital lease obligations, noncurrent			$ 195,785,000
6.875% Notes Due August 15, 2024 [Member]
Line of Credit Facility [Line Items]
Face amount of notes		$ 550,000,000
Notes payable, stated interest rate		6.875%	6.875%
Maturity date			Aug. 15, 2024
2016 Senior Unsecured Guaranteed Credity Facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity			$ 1,200,000,000
Credit facility, maturity date			Aug. 01, 2019
Line of credit facility fee			0.50%
Issue cost of debt facility		$ 14,000,000
Amount outstanding			$ 0
Minumum borrowing capacity through automatic reductions			$ 1,000,000,000
Minimum adjusted EBITDAX to consolidated interest expense			250.00%
Maximum consolidated debt to maximum adjusted EBITDAX			375.00%
2016 Senior Unsecured Guaranteed Credity Facility [Member] | United States And Canada [Member]
Line of Credit Facility [Line Items]
Minimum liquidity			$ 500,000,000
2016 Senior Unsecured Guaranteed Credity Facility [Member] | Scenario, Forecast [Member]
Line of Credit Facility [Line Items]
Maximum leverage ratio	325.00%
2016 Senior Unsecured Guaranteed Credity Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate above LIBOR			2.50%
2016 Senior Unsecured Guaranteed Credity Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate above LIBOR			4.50%
2016 Senior Unsecured Guaranteed Credity Facility [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Amount outstanding			$ 88,000,000
Amended 2016 Senior Unsecured Guaranteed Credity Facility [Member]
Line of Credit Facility [Line Items]
Threshold of outstanding borrowings on facility that triggers guarantee obligation by subsidiary					$ 500,000,000
Threshold of debt to EBITDAX that triggers guarantee obligation by subsidiary					425.00%
Amended 2016 Senior Unsecured Guaranteed Credity Facility [Member] | Scenario, Forecast [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity	$ 1,100,000,000
Threshold of debt to EBITDAX that triggers guarantee obligation by subsidiary				400.00%
2011 Unscured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity		$ 2,000,000,000				$ 630,000,000
Credit facility, maturity date			Jun. 01, 2017
Amount outstanding			$ 0
Credit facility interest rate above LIBOR			1.45%
Maximum total debt to total capital employed			60.00%