Equity-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended							3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Class B Common Unit [Member]		Dec. 31, 2012 Class B Common Unit [Member]		Mar. 10, 2010 Class B Common Unit [Member]	Mar. 31, 2013 MPK Plan Units [Member]		Dec. 31, 2012 MPK Plan Units [Member]
Equity-Based Compensation Plan Activity [Line Items]
Equity-based compensation expense	$ 1.9	$ 5.7
Equity-based compensation expense, incremental expense		1.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Number			216,092	[1]	216,483	[1]		64,121	[1],[2]	66,137	[1],[2]
Equity-Based Compensation Plan Activity [Roll Forward]
Equity awards granted			400	[1]				0	[1],[2]
Equity awards forfeited			(665)	[1]				(1,534)	[1],[2],[3]
Equity awards repurchased/settled			(126)	[1],[4]				(482)	[1],[2],[4]
Equity awards vested			125,903	[1]				112	[1],[2],[5]
Weighted average grant date fair market value, granted during period			$ 119.00
Weighted average grant date fair market value, outstanding units, impact of modifications							$ 60.00
Weighted average grant date fair market value, outstanding units			$ 279.58					$ 1,000.00
Equity Awards forfeited in period, future charitable contribution								1,534
Equity-based compensation expense, not yet recognized	$ 28.5
Equity-based compensation expense, not yet recognized, period for recognition	4 years 110 days

[1] The weighted-average grant date fair market value for Class B Common Units granted during the period ended March 31, 2013 is $119.00. The weighted-average grant date fair market value for outstanding Class B Common Units inclusive of the $60.00 per unit impact of the March 2010 modification and the impact of the June 2011 modification for the Company's former chief executive officer is $279.58. The weighted-average grant date fair market value for outstanding MPK Plan Units is $1,000.

[2] Represents units notionally credited to participants' accounts.

[3] The Company contributes the fair market value of awards forfeited under the plan to a charitable foundation. The contribution is generally made in the quarter following that in which the units are forfeited. As of March 31, 2013, the Company owed a contribution of 1,534 units

[4] Represents Class B Common Units that were repurchased by the Company from former participants and the settlement of vested MPK Plan Units through the issuance of Class A Common Units in exchange for the vested MPK Plan Units.

[5] Represents MPK Plan Units that have vested but not yet converted to Class A Common Units.