Debt - Credit Facility (Details) $ in Millions	3 Months Ended
	Dec. 31, 2015 USD ($) drilling_unit	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Weighted average interest rate	4.10%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Long-term line of credit, maximum borrowing capacity	$ 1,547
Long-term line of credit	960
Long-term line of credit, remaining borrowing capacity	587
Interest costs capitalized	$ 5	$ 4
Revolving Credit Facility | Minimum | Eurodollar
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Unused capacity commitment fee	0.30%
Revolving Credit Facility | Maximum | Eurodollar
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.00%
Unused capacity commitment fee	0.40%
Commercial Real Estate | Revolving Credit Facility
Line of Credit Facility [Line Items]
Number of drilling units | drilling_unit	8
Revolving Credit Facility, Maturing May 2018 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Long-term line of credit, maximum borrowing capacity	$ 305
Revolving Credit Facility, Maturing May 2019 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Long-term line of credit, maximum borrowing capacity	$ 1,242