The following summarizes impairments by segment for the years ended December 31:

millions			2012		2011		2010
Oil and gas exploration and production
	Long-lived assets held for use
		U.S. onshore properties	$	259	$	1,063	$	31
		Gulf of Mexico properties		104		162		114
	Cost-method investment			13		91		61
Midstream
	Long-lived assets held for use			13		458		2
Marketing
	Long-lived assets held for use			—		—		8
Impairments			$	389	$	1,774	$	216

In 2012, U.S. onshore and midstream properties were impaired primarily due to lower natural-gas prices. The Gulf of Mexico properties were impaired primarily as a result of downward reserves revisions for a property that was near the end of its economic life. In 2011, U.S. onshore and midstream properties were impaired primarily due to decreases in natural-gas prices, and Gulf of Mexico properties were impaired due to declines in estimated recoverable reserves. In 2010, the impairment of Gulf of Mexico properties related to a production platform that remains idle with no immediate plan for use, and for which a limited market exists. Impairments of the Company's Venezuelan cost-method investment were due to declines in estimated recoverable reserves in 2012, 2011, and 2010, and lower crude-oil prices in 2012.

The following summarizes the aggregate fair values of the above-described assets, by major category and input level within the fair-value hierarchy, at the respective dates of impairment:


millions
2012	Level 1		Level 2⁽¹⁾		Level 3⁽¹⁾		Total
Long-lived assets held for use	$	—	$	—	$	103	$	103
Cost-method investment		—		—		34		34

2011
Long-lived assets held for use	$	—	$	187	$	739	$	926
Cost-method investment		—		—		39		39

Impairments of Unproved Properties Impairments of unproved properties are included in exploration expense in the Company's Consolidated Statements of Income. In 2012, the Company recognized a $721 million impairment of unproved Powder River coalbed methane properties primarily resulting from lower natural-gas prices. Also in 2012, the Company recognized a $124 million impairment of an unproved Gulf of Mexico natural-gas property that the Company does not plan to pursue under the forecasted natural-gas price environment.