Equity-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2010 Class B Common Unit [Member]	Dec. 31, 2012 Class B Common Unit [Member]		Dec. 31, 2011 Class B Common Unit [Member]		Dec. 31, 2010 Class B Common Unit [Member]	Mar. 10, 2010 Class B Common Unit [Member]	Dec. 31, 2012 MPK Plan Units [Member]		Dec. 31, 2010 MPK Plan Units [Member]	Oct. 15, 2007 MPK Plan Units [Member]
Equity-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units Authorized					250,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period									5 years 0 months 0 days
Share-based Compensation Arrangement, Award Vesting Qualifications, Age														62
Share-based Compensation Arrangement, Award Vesting Qualification, Years of Service														10 years 0 months 0 days
Equity-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Equity-based Compensation Plan Modification, Incremental Compensation Cost				$ 8.4	$ 6.6		$ 3.3
Equity-based Compensation Plan Modification, Participation Threshold post Modification										$ 0.01
Equity-based Compensation Plan Modification, Participation Threshold prior to Modification										$ 1,000
Equity-based Compensation Plan Modification, Number of Units Affected				140,428
Equity-based Compensation Plan Modification, Number of Employees Affected				101
Weighted-Average Grant Date Fair Value, Impact of Modification										$ 60.00
Per Unit Fair Value post Modification										$ 120.00
Per Unit Fair Value before Modification										$ 60.00
Equity-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Weighted-Average Grant Date Fair Value					$ 125.65		$ 148.89		$ 130.45					$ 1,000.00
Weighted-Average Volatility					65.26%		82.87%		97.86%
Weighted-Average Risk-Free Rate					0.19%		0.84%		2.32%
Dividend Yield					0.00%		0.00%		0.00%
Equity-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Equity Awards Outstanding, Beginning of Year					202,908	[1]					70,113	[1],[2]
Equity Awards Granted					16,008	[1]					0	[1],[2]
Equity Awards Forfeited					(1,615)	[1]					(3,366)	[1],[2],[3]
Equity Awards Repurchased/Settled					(818)	[1],[4]					(610)	[1],[2],[4]
Equity Awards Outstanding, End of Period					216,483	[1]	202,908	[1]			66,137	[1],[2]
Equity Awards Vested					115,198	[1]					450	[1],[2],[5]
Weighted-Average Grant Date Fair Value, Outstanding Units					$ 260.26						$ 1,000.00
Equity Awards Forfeited in Period Future Charitable Contribution											777
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract]
Equity-based Compensation Expense Not yet Recognized	30.8
Equity-based Compensation Expense Not yet Recognized, Period for Recognition	4 years 178 days
Equity-based compensation [Abstract]
Equity-based compensation expense	22.1	19.5	11.5
Equity-based Compensation Expense, Tax Benefit from Compensation Expense	2.3	1.9	0.1
Equity-based Compensation Expense, Capitalized Amount	0	0	0
Change in Equity-based Compensation Expense, Due to Change in Cumulative Forfeiture Rate													$ 5.3

[1] The weighted-average grant date fair market value for Class B Common Units granted during the period ended December 31, 2012 is $125.65. 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 Mr. Edwardson is $260.26. 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 December 31, 2012, the Company owed a contribution for 777 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.