Equity-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011
Equity-Based Compensation Plan Activity [Line Items]
Equity-based compensation expense	$ 5.8		$ 4.1	$ 11.5		$ 8.1
Equity-based compensation expense, incremental expense	1.7			3.3
Equity-Based Compensation Plan Activity [Roll Forward]
Equity-based compensation expense, not yet recognized	$ 37.7			$ 37.7
Equity-based compensation expense, not yet recognized, period for recognition				4 years 73 days
Class B Common Unit [Member]
Equity-Based Compensation Plan Activity [Roll Forward]
Equity awards outstanding				202,908	[1]
Equity awards granted				1,520	[1]
Equity awards forfeited				(957)	[1]
Equity awards repurchased/settled				(325)	[1]
Equity awards outstanding	203,146	[1]		203,146	[1]
Equity awards vested	95,376	[1]		95,376	[1]
Weighted average grant date fair market value, granted during period				$ 189.00
Weighted average grant date fair market value, outstanding units, impact of modifications	$ 60.00			$ 60.00
Weighted average grant date fair market value, outstanding units	$ 291.28			$ 291.28
MPK Plan Units [Member]
Equity-Based Compensation Plan Activity [Roll Forward]
Equity awards outstanding				70,113	[1],[2]
Equity awards granted				0	[1],[2]
Equity awards forfeited				(1,691)	[1],[2],[3]
Equity awards repurchased/settled				(610)	[1],[2],[4]
Equity awards outstanding	67,812	[1],[2]		67,812	[1],[2]
Equity awards vested	197	[1],[2],[5]		197	[1],[2],[5]
Weighted average grant date fair market value, outstanding units	$ 1,000.00			$ 1,000.00
Equity Awards forfeited in period, current charitable contribution				981
Equity Awards forfeited in period, future charitable contribution				710

[1] The weighted-average grant date fair market value for Class B Common Units granted during the period ended June 30, 2012 is $189.00. 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 $291.28. 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. The Company made a contribution for 981 units forfeited during the three months ended March 31, 2012. The Company will make a future contribution with respect to 710 units that were forfeited during the three months ended June 30, 2012.

[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 Units in exchange for the vested MPK Plan Units.

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