Equity-based Awards, Phantom Unit Awards (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]
Cash payments made in connection with DERs	$ 1.2		$ 0
Phantom Unit Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate of phantom unit awards (in hundredths)	25.00%
Summary of awards activity, equity instruments other than options [Roll Forward]
Beginning of period (in units)	3,342,390
Granted (in units)	3,446,240	[1]
Vested (in units)	(786,890)
Forfeited (in units)	(78,204)
End of period (in units)	5,923,536
Phantom units outstanding, weighted-average grant date fair value [Roll Forward]
Weighted-average grant date fair value per unit, at beginning of period (in dollars per unit)	$ 33.13	[2]
Granted weighted-average grant date fair value per unit (in dollars per unit)	$ 34.05	[1],[2]
Vested weighted-average grant date fair value per unit (in dollars per unit)	$ 33.04	[2]
Forfeited weighted-average grant date fair value per unit (in dollars per unit)	$ 33.16	[2]
Weighted-average grant date fair value per unit, at end of period (in dollars per unit)	$ 33.67	[2]
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]
Aggregate grant date fair value	117.3
Estimated forfeiture rate (in hundredths)	3.50%
Cash payments made in connection with DERs	1.2		0
Total intrinsic value of phantom unit awards that vested during period	26.6		0
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost	146.9
Recognition period for total unrecognized compensation cost	2 years 3 months 18 days
Phantom Unit Awards [Member] | Minimum [Member]
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]
Grant date market price of common units (in dollars per unit)	$ 34.04
Phantom Unit Awards [Member] | Maximum [Member]
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]
Grant date market price of common units (in dollars per unit)	$ 34.40
Phantom Unit Awards [Member] | Enterprise [Member]
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost	$ 136.5

[1] The aggregate grant date fair value of phantom unit awards issued during 2015 was $117.3 million based on a grant date market price of our common units ranging from $34.04 to $34.40 per unit. An estimated annual forfeiture rate of 3.5% was applied to these awards.

[2] Determined by dividing the aggregate grant date fair value of awards (before an allowance for forfeitures) by the number of awards issued.