Capitalized Exploratory Well Costs (Tables)

12 Months Ended

Dec. 31, 2016

Capitalized Exploratory Well Costs [Abstract]

Capitalized Exploratory Well Costs, Roll Forward [Table Text Block]

	Capitalized Exploratory Well Costs
	2016			2015			2014
	(in millions)
Balance at January 1,	$	2.6		$	12.6		$	2.6
Additions to capitalized exploratory well costs pending the determination of proved reserves	11.7			6.0			13.7
Reclassifications to proved properties after the determination of proved reserves	—			(16.0		)	—
Capitalized exploratory well costs charged to expense	(0.1		)	—			(3.7		)
Balance at December 31,	$	14.2		$	2.6		$	12.6