Stock-based compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended				1 Months Ended	0 Months Ended	1 Months Ended		0 Months Ended	1 Months Ended					6 Months Ended		6 Months Ended				1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Preferred and Class A Units CFO	Jun. 30, 2013 Preferred and Class A Units COO	Jun. 30, 2012 Preferred Unit awards CFO	May 10, 2013 Preferred Unit awards COO	Jun. 30, 2012 Preferred Unit awards COO	Jun. 30, 2012 Class A awards CFO	May 10, 2013 Class A awards COO	Jun. 30, 2012 Class A awards COO	Feb. 28, 2008 2008 Plan	Feb. 28, 2008 2008 Plan Class B Units	Feb. 28, 2008 2008 Plan Class C Units	Jun. 11, 2012 2012 Plan	Jun. 30, 2013 2012 Plan MEP Units	Jun. 11, 2012 2012 Plan MEP Units	Jun. 30, 2013 2012 Plan MEP Units Newly hired individuals	Jun. 30, 2013 2012 Plan EMEP Units item	Jun. 11, 2012 2012 Plan EMEP Units	Jun. 11, 2012 2012 Plan Class L Units	Jun. 30, 2012 2012 Plan Class L Units CEO	Jun. 30, 2013 2012 Plan Class G Units	Jun. 11, 2012 2012 Plan Class G Units
Stock-based compensation
Number of units authorized for issuance (in shares)													1,494,111	526,262	967,849	1,100,200		1,000,000			100,000	100			100
Number of units available for future issuance (in shares)																	125,543			24,072				100
Vesting percentage upon issuance						33.30%											66.60%
Vesting percentage on the 18 month anniversary of the issuance date																	33.40%
Vesting percentage in years one and two of the hire date																			33.30%
First year from the hire date																			1 year
Second year from the hire date																			2 years
Vesting percentage in year three of the hire date																			33.40%
Third year from the hire date																			3 years
Vesting right if the implied equity value exceeds a specified threshold (as a percent)																				25.00%
Minimum number of consecutive quarters for which implied value should exceed several higher thresholds for incremental vesting eligibility																				2
Incremental vesting right if implied value exceeds several higher thresholds for a specified minimum number of quarters (as a percent)																				50.00%
Stock- based compensation expense	$ 0.2	$ 2.9	$ 0.3	$ 3.0																$ 0			$ 1.0
Assumptions used to determine the fair value of units at the time of grant
Assumed forfeiture rate (as a percent)																	3.90%
Requisite service period																	18 months			36 months
Nonvested Activity, Units Outstanding
Nonvested balance at beginning of period (in shares)																	349,356			90,743
Units granted (in shares)							296		500	5,625		25,000					4,545
Units forfeited (in shares)								(167)			(8,333)						(45,546)			(14,815)
Units vested (in shares)																	(3,027)
Nonvested balance at end of period (in shares)																	305,328			75,928
Nonvested Activity, Weighted-Average Grant Date Fair Value
Nonvested balance at beginning of period (in dollars per share)																	$ 3.32			$ 38.94
Units granted (in dollars per share)																	$ 0.93
Units forfeited (in dollars per share)																	$ 3.32			$ 38.94
Units vested (in dollars per share)																	$ 0.93
Nonvested balance at end of period (in dollars per share)																	$ 3.31			$ 38.94
Total unrecognized compensation expense																	$ 0.4			$ 2.8
Weighted-average period of recognition																	10 months 24 days
Vesting percentage on January 1, 2013					33.30%
Vesting percentage in equal amounts at next two anniversary dates					66.70%	66.70%