Disclosures about fair value measurements (Tables)

6 Months Ended

Jun. 30, 2017

Disclosure Fair Value Narrative [Abstract]

Carrying amounts and fair values of the Company's financial instruments

The following table presents the carrying amounts and fair values of the Company’s financial instruments at June 30,2017 and December 31, 2016:

			June 30, 2017				December 31, 2016
			Carrying		Fair		Carrying		Fair
(in millions)			Value		Value		Value		Value
	Assets:
		Derivative instruments	$	225	$	225	$	4	$	4
	Liabilities:
		Derivative instruments	$	-	$	-	$	178	$	178
		$600 million 5.5% senior notes due 2022 (a)	$	594	$	619	$	594	$	620
		$1,550 million 5.5% senior notes due 2023 (a)	$	1,554	$	1,597	$	1,555	$	1,621
		$600 million 4.375% senior notes due 2025 (a)	$	593	$	614	$	592	$	599
(a)	The carrying value includes associated deferred loan costs and any premium.

Net basis derivative fair values as reported in the consolidated balance sheets

The following tables summarize (i) the valuation of each of the Company’s financial instruments by required fair value hierarchy levels and (ii) the gross fair value by the appropriate balance sheet classification,even when the derivative instruments are subject to netting arrangements and qualify for net presentation in the Company’s consolidated balance sheets at June 30,2017 and December 31, 2016. The Company nets the fair value of derivative instruments by counterparty in the Company’s consolidated balance sheets.

June 30, 2017
				Fair Value Measurements Using												Net
				Quoted Prices										Gross		Fair Value
				in Active				Significant						Amounts		Presented
				Markets for				Other		Significant				Offset in the		in the
				Identical				Observable		Unobservable				Consolidated		Consolidated
				Assets				Inputs		Inputs		Total		Balance		Balance
(in millions)				(Level 1)				(Level 2)		(Level 3)		Fair Value		Sheet		Sheet
	Assets:
		Current:
			Commodity derivatives		$	-	$	145	$	-	$	145	$	(10)	$	135
		Noncurrent:
			Commodity derivatives			-		94		-		94		(4)		90
	Liabilities:
		Current:
			Commodity derivatives			-		(10)		-		(10)		10		-
		Noncurrent:
			Commodity derivatives			-		(4)		-		(4)		4		-
	Net derivative instruments				$	-	$	225	$	-	$	225	$	-	$	225

December 31, 2016
				Fair Value Measurements Using												Net
				Quoted Prices										Gross		Fair Value
				in Active				Significant						Amounts		Presented
				Markets for				Other		Significant				Offset in the		in the
				Identical				Observable		Unobservable				Consolidated		Consolidated
				Assets				Inputs		Inputs		Total		Balance		Balance
(in millions)				(Level 1)				(Level 2)		(Level 3)		Fair Value		Sheet		Sheet
	Assets:
		Current:
			Commodity derivatives		$	-	$	59	$	-	$	59	$	(55)	$	4
		Noncurrent:
			Commodity derivatives			-		-		-		-		-		-
	Liabilities:
		Current:
			Commodity derivatives			-		(137)		-		(137)		55		(82)
		Noncurrent:
			Commodity derivatives			-		(96)		-		(96)		-		(96)
	Net derivative instruments				$	-	$	(174)	$	-	$	(174)	$	-	$	(174)

Carrying amounts, estimated fair values and impairment expense of long-lived assets

The following table reports the carrying amount, estimated fair value and impairment expense of long-lived assets for the indicated period:

				Estimated
		Carrying		Fair Value		Impairment
(in millions)		Amount		(Level 3)		Expense
March 2016	$	3,438	$	1,913	$	1,525