UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Allowance for doubtful accounts	$ 11.9	$ 13.2
Accumulated amortization	$ 1,075.7	$ 1,050.0
Common units [Member]
Limited partners:
Capital account, units outstanding (in units)	910,805,527	898,813,337
Class B units [Member]
Limited partners:
Capital account, units outstanding (in units)	4,520,431	4,520,431