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Shareholders' Equity / Partners' Capital - Restricted Stock Units (Details) - Restricted stock units - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock units at beginning of period (in shares) 130,150 91,650 85,000
Units awarded (in shares) 174,400 74,100 44,000
Units vested (in shares) (42,475) (27,250) (22,000)
Units forfeited (in shares) (18,200) (8,350) (15,350)
Restricted stock units at end of period (in shares 243,875 130,150 91,650