UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Accounts receivable, allowance for doubtful accounts	$ 11.8	$ 12.1
Intangible assets, accumulated amortization	$ 1,651.5	$ 1,564.8
Limited partners:
Common units outstanding (in units)	2,175,951,128	2,161,089,479