Equity method investments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Aug. 31, 2012 ECTE [Member]	Nov. 30, 2011 ECTE [Member]	Nov. 30, 2010 ECTE [Member]	Nov. 30, 2010 ENTE [Member]	Nov. 30, 2010 ERTE [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership interest in equity method investment sold						59.96%	100.00%	100.00%
Number of years over which remaining ownership interest in equity method investment will be sold						4 years
Portion of remaining interest sold				25.00%	25.00%
Total assets of equity method investments	$ 114.8	$ 129.0	$ 104.4
Total long-term debt of equity method investments	56.2	65.5	30.3
Investment in equity method investments	5.5	6.9	7.4
Undistributed earnings of equity method investments	$ 2.0	$ 3.4	$ 2.3