Short-Term Borrowings (Schedule Of Balances Outstanding And Interest Rates Of Borrowings) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Balance outstanding at end of period	$ 82,000	$ 61,000
Letters of credit outstanding at end of period	26,815	29,030
Average interest rate at end of period	1.08%	1.12%
Ecova [Member]
Line of Credit Facility [Line Items]
Balance outstanding at end of period	$ 58,000	$ 35,000
Average interest rate at end of period	2.49%	2.38%