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Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Capitalized Cost [Abstract]
Proved properties	$ 11,571.4	$ 10,234.3
Unproved properties, net	665.1	937.9
Total proved and unproved properties	12,236.5	11,172.2
Accumulated depreciation, depletion and amortization	(4,930.9)	(4,258.1)
Net capitalized costs	$ 7,305.6	$ 6,914.1

Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table.

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- Definition

Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table.

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