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Property, Plant and Equipment - Capitalized Costs of Unproved Properties Excluded from Amortization (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2013
Year Cost Incurred 2013
Dec. 31, 2012
Year Cost Incurred 2012
Dec. 31, 2011
Year Cost Incurred 2011
Dec. 31, 2010
Year Cost Incurred 2010 and Prior
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]          
Property acquisition, cumulative $ 515,849       $ 75,434
Property acquisition   114,252 291,999 34,164  
Exploration, cumulative 61,327 [1]       3,486 [1]
Exploration   48,764 [1] 5,479 [1] 3,598 [1]  
Total costs incurred, cumulative 577,176       78,920
Total costs incurred   $ 163,016 $ 297,478 $ 37,762  
[1] Includes $45.6 million of pipe inventory costs incurred ($42.6 million in 2013, $2.8 million in 2012 and $0.2 million in 2010 and prior years).