Acquisitions and Divestitures (Tables)

12 Months Ended

Dec. 31, 2015

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

Business Acquisition, Pro Forma Information [Table Text Block]

	Year ended December 31,
	2014				2013
	Actual		Pro Forma		Actual		Pro Forma
	(in millions, except per share amounts)
Revenues	$	3,293.2	$	3,319.3	$	2,685.1	$	2,858.8
Net income	784.4		791.4		159.4		195.3
Earnings per common share
Basic	$	4.36	$	4.40	$	0.89	$	1.09
Diluted	4.36		4.40		0.89		1.09