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Long-Term Debt and Commercial Paper (Restrictions And Covenants) (Details) - Credit Facility Due 2019 [Member]
$ in Millions
Mar. 31, 2016
USD ($)
Debt Instrument [Line Items]  
Capitalization ratio, requirement 70.00%
Leverage ratio, requirement 3.75
The maximum threshold of letters of credit excluded from the leverage ratio calculation $ 150.0
Capitalization ratio calculation, additions to shareholders' equity $ 1,530.0