Capitalized Exploratory Well Costs (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)
Capitalized Exploratory Well Costs [Abstract]
Capitalized exploratory well costs, beginning balance	$ 14.2
Additions to capitalized exploratory well costs pending the determination of proved reserves	10.0
Capitalized exploratory well costs, ending balance	24.2
Capitalized exploratory well costs that have been capitalized for a period of one year or less	10.4
Capitalized exploratory well costs that have been capitalized for a period greater than one year	$ 13.8
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	1
Project with exploratory well costs capitalized for more than one year, unproved leasehold costs	28.6