CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Allowance for doubtful accounts	$ 12.1	$ 13.9
Accumulated amortization	$ 1,235.8	$ 1,246.3
Limited partners:
Capital account, units outstanding (in units)	2,012,553,024	1,937,324,817