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Equity-Based Compensation, Assumptions (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Feb. 29, 2012	Apr. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected volatility	30.00%	41.00%
Expected forfeiture rate	0.00%	0.00%
Risk-free interest rate	0.34%	0.79%
Unit price on date of grant (dollars per unit)			$ 33.38	$ 23.11

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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