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Partners' Capital, Equity and Incentive Compensation Plans (Schedule of Quarterly Cash Dividends) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Apr. 15, 2018
Apr. 05, 2018
Mar. 22, 2018
Mar. 15, 2018
Mar. 31, 2018
Mar. 31, 2017
Record Date     Mar. 22, 2018   Dec. 05, 2017 Dec. 16, 2016
Payment Date   Apr. 05, 2018     Jan. 05, 2018 Jan. 05, 2017
Per Common Share and Per Unit Rate   $ 0.41     $ 0.39 $ 0.36
Dividend/Distribution Amount         $ 22,200 $ 19,700
Dividend rate (as a percent)       7.125%    
Preferred stock, issued       4,280,000    
Preferred Stock issuance proceeds, net of costs         $ 103,615  
Preferred stock dividends per share cash paid $ 0.14844          
Preferred stock redemption price per share         $ 25.00 $ 25.00
Threshold period of redemption of preferred stock         120 days  
Ownership interest         50.00%  
Convertible preferred stock par value           $ 0.01
Preferred stock, liquidation preference       $ 25.00 $ 25.00  
Share cap price         $ 1.46929  
Underwriter's Option            
Preferred stock, issued       280,000