CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	$ 156,569	$ 139,620
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	87,583	78,453
Recognition, refund and collection of revenue accruals and deferrals — including accrued interest	30,369	25,748
Deferred income tax expense	69,733	44,921
Allowance for equity funds used during construction	(25,315)	(15,800)
Other	13,113	9,030
Changes in assets and liabilities, exclusive of changes shown separately:
Accounts receivable	(31,115)	(12,182)
Inventory	1,075	979
Prepaid and other current assets	11,208	(5,776)
Accounts payable	4,044	(10,637)
Accrued payroll	(1,707)	(1,865)
Accrued interest	(14,538)	(247)
Accrued taxes	(10,467)	(5,773)
Other current liabilities	7,979	11,474
Other non-current assets and liabilities, net	(6,328)	410
Net cash provided by operating activities	292,203	258,355
CASH FLOWS FROM INVESTING ACTIVITIES
Expenditures for property, plant and equipment	(630,485)	(637,386)
Proceeds from sale of securities	42,174	5,935
Purchases of securities	(43,398)	(10,786)
Other	(3,340)	(747)
Net cash used in investing activities	(635,049)	(642,984)
CASH FLOWS FROM FINANCING ACTIVITIES
Issuance of long-term debt	933,025	100,000
Borrowings under revolving credit agreements	890,400	1,073,550
Borrowings under term loan credit agreements	535,000	200,000
Retirement of long-term debt	(452,000)	0
Repayments of revolving credit agreements	(1,004,100)	(960,350)
Repayments of term loan credit agreements	(475,000)	0
Issuance of common stock	8,213	4,929
Dividends on common and restricted stock	(61,821)	(55,677)
Refundable deposits from generators for transmission network upgrades	26,898	31,157
Repayment of refundable deposits from generators for transmission network upgrades	(30,279)	(31,186)
Other	(2,973)	(6,112)
Net cash provided by financing activities	367,363	356,311
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	24,517	(28,318)
CASH AND CASH EQUIVALENTS — Beginning of period	26,187	58,344
CASH AND CASH EQUIVALENTS — End of period	$ 50,704	$ 30,026