Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 2.4	$ 1.9
Limited partners' units, issued	70,721,563	69,499,741
Limited partners' units, outstanding	70,721,563	69,499,741
Preferred units, issued	71,257,445	66,533,415
Preferred units, outstanding	71,257,445	66,533,415
CMLP
Allowance for doubtful accounts	$ 2.4	$ 1.9