Acquisitions & Divestitures (Tables)

9 Months Ended

Sep. 30, 2015

Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]

Business Combination, Separately Recognized Transactions [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]

		Nine Months Ended
		September 30, 2014
		Actual		Pro forma
	(in millions, except per share data)
Revenues		$	2,614.7	$	2,640.8
Net income		$	118.5	$	125.5
Earnings per common share
Basic		$	0.66	$	0.70
Diluted		$	0.66	$	0.70