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) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	3.08%	3.14%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	$ 48.63	$ 45.78	$ 35.81
Fair value of options granted	$ 10.11	$ 9.57	$ 8.00
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Expected volatility	28.00%	30.70%	33.00%
Expected dividend yield			3.40%
Expected risk-free interest rates	2.12%	1.42%	1.67%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	$ 51.88	$ 56.28
Fair value of options granted	$ 10.36	$ 9.97	$ 8.77
Expected term (years)	5 years 10 months 24 days	5 years 10 months 24 days	6 years 1 month 6 days
Expected volatility		31.30%
Expected dividend yield			3.57%
Expected risk-free interest rates	2.18%	1.48%	1.94%