Derivatives And Risk Management (Narrative) (Detail) (USD $)	1 Months Ended	6 Months Ended
	May 30, 2012	Jun. 30, 2012 Day	Dec. 31, 2011
Derivative [Line Items]
Number of days Canadian currency prices are settled with U.S. dollars		60
Notional amount	$ 75,000,000
Cash settlement on interest rate swap	18,500,000
First mortgage bonds	80,000,000
Cash deposited as collateral		19,000,000
Letters of credit outstanding		25,075,000	29,030,000
Cash collateral against net derivative positions		15,000,000
Aggregate fair value of all derivative instruments that are in a liability position		155,000,000
Collateral agreements with counterparties		51,500,000
Energy Related Derivatives [Member]
Derivative [Line Items]
Letters of credit outstanding		$ 17,500,000