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, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility		28.00%
Expected dividend yield	4.82%	3.08%	3.14%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	$ 34.03	$ 48.63	$ 45.78
Fair value of options granted	$ 5.55	$ 10.11	$ 9.57
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Expected volatility	28.00%	28.00%	30.70%
Expected dividend yield			3.14%
Expected risk-free interest rates	2.69%	2.12%	1.42%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of restricted stock granted	$ 54.01	$ 51.88
Fair value of options granted	$ 5.64	$ 10.36	$ 9.97
Expected term (years)	6 years	5 years 10 months 24 days	5 years 10 months 24 days
Expected volatility			31.30%
Expected risk-free interest rates	2.73%	2.18%	1.48%