Partners' Capital, Equity and Incentive Compensation Plans (Summary of Assumptions and Fair Values for Restricted Stock and Options to Purchase Shares of Class A Common Stock Granted) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	33.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	35.81
Fair value of options granted	8.00
Expected term (years)	5 years 6 months
Expected dividend yield	3.50%
Expected risk-free interest rates	1.67%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	37.64
Fair value of options granted	8.05
Expected term (years)	6 years 1 month 6 days
Expected dividend yield	3.57%
Expected risk-free interest rates	1.69%