Acquisitions and Divestitures (Tables)

12 Months Ended

Dec. 31, 2016

Business Combinations [Abstract]

Schedule of Purchase Price Allocation [Table Text Block]

Consideration:
Total consideration	$	941.8
Amounts recognized for fair value of assets acquired and liabilities assumed:
Proved properties	$	472.1
Unproved properties	480.6
Asset retirement obligations	(9.7		)
Liabilities assumed	(1.2		)
Total fair value	$	941.8

Consideration:
Total consideration	$	590.6
Amounts recognized for fair value of assets acquired and liabilities assumed:
Proved properties	$	406.2
Unproved properties	214.2
Asset retirement obligations	(11.6		)
Bargain purchase gain	(18.2		)
Total fair value	$	590.6

Business Acquisition, Pro Forma Information [Table Text Block]

	Year ended December 31,
	2016						2015
	Actual			Pro forma			Actual			Pro forma
	(in millions, except per share amounts)
Revenues	$	1,377.1		$	1,392.5		$	2,018.6		$	2,041.5
Net income (loss)	$	(1,245.0	)	$	(1,246.8	)	$	(149.4	)	$	(152.5	)
Earnings (loss) per common share
Basic	$	(5.62	)	$	(5.62	)	$	(0.85	)	$	(0.86	)
Diluted	$	(5.62	)	$	(5.62	)	$	(0.85	)	$	(0.86	)

	Year ended December 31,
	2014
	Actual		Pro Forma
	(in millions, except per share amounts)
Revenues	$	3,293.2	$	3,319.3
Net income (loss)	$	784.4	$	791.4
Earnings (loss) per common share
Basic	$	4.36	$	4.40
Diluted	$	4.36	$	4.40