Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 lease	Dec. 31, 2010	Dec. 31, 2009
Leases
Types of leases	2
Leases Disclosures
Aggregate lease expense	$ 322.8	$ 258.1	$ 422.8
Future minimum payments for long-term, noncancelable, operating leases:
2012	230.2
2013	220.5
2014	227.6
2015	213.1
2016	121.1
Thereafter	69.0
Total future minimum lease payments	1,081.5
Time charter purchase agreements for the use of dry bulk freight vessels
Leases Disclosures
Aggregate lease expense	7.0	11.0	145.0
Dry bulk freight vessels hired under operating lease (in units)	3	12	31
Power Purchase Agreements
Future minimum payments for long-term, noncancelable, operating leases:
2012	198.2
2013	190.9
2014	193.8
2015	193.3
2016	108.2
Thereafter	51.9
Total future minimum lease payments	936.3
Other.
Future minimum payments for long-term, noncancelable, operating leases:
2012	32.0
2013	29.6
2014	33.8
2015	19.8
2016	12.9
Thereafter	17.1
Total future minimum lease payments	145.2
Nonregulated Revenues | Time charter purchase agreements for the use of dry bulk freight vessels
Sub-Lease Arrangements
Time charter sub-lease income	2	25	114
Fuel and purchased energy expenses
Leases Disclosures
Aggregate lease expense	291.1	227.9	385.6
Operating expenses
Leases Disclosures
Aggregate lease expense	$ 31.7	$ 30.2	$ 37.2