v2.4.0.6

Summary of significant accounting policies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Deferred financing costs |  |  |
Net carrying amount of deferred financing cost | $ 47.2 | $ 40.7 |
Amortization expenses | 4.4 | 1.3 | 1.2
Property, plant and equipment |  |  |
Estimated useful lives | 45 years |  |
Project Development Costs and Capitalized Interest |  |  |
Capitalized interest | $ 17.0 | $ 3.0 | $ 5.0
Transmission system rights |  |  |
Transmission system rights |  |  |
Right to use percentage of Path 15 transmission line in California | 72.00% |  |
Regulatory life | 30 years |  |
Maximum |  |  |
Deferred financing costs |  |  |
Amortization period | 28 years |  |
Impairment of long-lived assets, non-amortizing intangible assets and equity method investments |  |  |
Maximum percentage of ownership in entities in which consolidation is not required | 50.00% |  |
Investments accounted for by the equity method |  |  |
Percentage of investments in partnerships and limited liability companies in which the entity is having the ability to exercise significant influence over operating and financial policies | 50.00% |  |
Maximum | Capitalized maintenance costs |  |  |
Property, plant and equipment |  |  |
Estimated useful lives | 6 years |  |
Minimum |  |  |
Deferred financing costs |  |  |
Amortization period | 5 years |  |
Investments accounted for by the equity method |  |  |
Percentage of investments in partnerships and limited liability companies in which the entity is having the ability to exercise significant influence over operating and financial policies | 5.00% |  |
Minimum | Capitalized maintenance costs |  |  |
Property, plant and equipment |  |  |
Estimated useful lives | 3 years |  |

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