Recent Financing Transactions Recent Financing Transactions - Long-Term Debt (Details) In Millions, unless otherwise specified	Mar. 31, 2015 Solar Star Funding, LLC [Member] Solar Star Funding LLC Series B Senior Secured Notes 3.95% due June 2016 through June 2035 [Member] USD ($)	Mar. 31, 2015 AltaLink Investments, L.P. [Member] Altalink Investments 15-1 Series Bonds 2.244% due March 2022 [Member] CAD	Apr. 30, 2015 Subsequent Event [Member] Northern Powergrid (Yorkshire) plc [Member] Northern Powergrid (Yorkshire) 2.50% Bonds due April 2025 [Member] GBP (£)
Debt Instrument [Line Items]
Debt instrument, face amount	$ 325	200	£ 150
Debt instrument, interest rate, stated percentage	3.95%	2.244%	2.50%