Common Stock and Related Matters (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended					1 Months Ended		0 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014		Mar. 31, 2015		Feb. 27, 2015
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Value of shares issued in exchange for ATLS common units	$ 0.1			$ 0		$ 0.1
Common stock closing market price (in dollars per share)								$ 99.58
Dividends Payable [Line Items]
Amount of dividend paid	32.4		25.5
Dividend Declared, Q1 2015 [Member]
Dividends Payable [Line Items]
Date paid or to be paid	May 18, 2015
Total dividend declared	46.6
Amount of dividend paid	46.4
Accrued dividends	0.2	[1]				0.2	[1]
Dividend declared per share of common stock (in dollars per share)	$ 0.83000
Dividend Declared, Q4 2014 [Member]
Dividends Payable [Line Items]
Date paid or to be paid				Feb. 17, 2015
Total dividend declared				32.6
Amount of dividend paid				32.4
Accrued dividends				0.2	[1]
Dividend declared per share of common stock (in dollars per share)				$ 0.77500
Common Stock [Member]
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Number of shares issued in public offering (in shares)	3,738,000					3,250,000
Common stock issued price per share (in dollars per share)	$ 91					$ 91
Common stock net proceeds of public offering						292.8
Number of additional shares sold in public offering (in shares)						487,500
Additional net proceeds from public offering pursuant to exercise of underwriters' overallotment option						43.9
Payments to capital contribution in partnership						52.4
General partnership interest (in hundredths)						2.00%
Atlas Energy [Member]
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Number of shares issued in exchange for ATLS common units (in shares)								10,126,532
Value of shares issued in exchange for ATLS common units								$ 1,000.0
Replacement restricted stock units (in shares)								81,740

[1] Represents accrued dividends on restricted stock units that are payable upon vesting.