Property, Plant and Equipment (Additional Information) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Oil and gas proved properties
Property, Plant and Equipment [Line Items]
Cumulative capitalized interest	$ 38.1	$ 23.4
Non-oil and natural gas equipment
Property, Plant and Equipment [Line Items]
Cumulative capitalized interest	4.3	4.3
Building and structures
Property, Plant and Equipment [Line Items]
Cumulative capitalized interest	$ 17.1	$ 12.0