CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net income	$ 455,188	$ 453,383	$ 458,076
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	383,992	365,713	344,226
Demand side management program amortization	4,331	4,802	5,299
Deferred income taxes	227,823	316,253	38,092
Amortization of investment tax credits	(2,941)	(2,935)	(2,957)
Allowance for equity funds used during construction	(46,784)	(33,173)	(16,354)
Provision for bad debts	17,005	16,784	16,323
Pretax charge for project costs	0	0	10,666
Net realized and unrealized hedging and derivative transactions	(2,578)	(3,571)	(39,338)
Changes in operating assets and liabilities:
Accounts receivable	(42,921)	5,089	(58,226)
Accrued unbilled revenues	(23,132)	14,707	18,920
Inventories	(972)	(14,857)	30,203
Prepayments and other	(81,715)	(7,210)	(3,466)
Accounts payable	(22,789)	59,361	(56,176)
Net regulatory assets and liabilities	130,499	108,400	(31,776)
Other current liabilities	5,284	16,561	34,305
Pension and other employee benefit obligations	(38,905)	(48,886)	(57,974)
Change in other noncurrent assets	5,537	3,862	(10,101)
Change in other noncurrent liabilities	(19,130)	17,191	(1,267)
Net cash provided by (used in) operating activities	947,792	1,271,474	678,475
Investing activities
Utility capital/construction expenditures	(1,114,338)	(1,066,700)	(873,383)
Allowance for equity funds used during construction	46,784	33,173	16,354
Investments in utility money pool arrangement	(603,000)	(1,495,000)	(1,000,000)
Repayments from utility money pool arrangement	659,000	1,423,000	1,052,000
Net cash provided by (used in) investing activities	(1,011,554)	(1,105,527)	(805,029)
Financing activities
Proceeds from (repayments of) short-term borrowings, net	382,000	(154,000)	154,000
Borrowings under utility money pool arrangement	333,000	14,000	36,000
Repayments under utility money pool arrangement	(333,000)	(14,000)	(36,000)
Proceeds from issuance of long-term debt	295,598	492,313	790,379
Repayments of long-term debt	(275,000)	(250,000)	(648,750)
Capital contributions from parent	81,498	25,621	99,283
Dividends paid to parent	(433,788)	(263,942)	(266,971)
Net cash provided by (used in) financing activities	50,308	(150,008)	127,941
Net change in cash and cash equivalents	(13,454)	15,939	1,387
Cash and cash equivalents at beginning of period	21,089	5,150
Cash and cash equivalents at end of period	7,635	21,089	5,150
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(150,011)	(155,457)	(179,610)
Cash received (paid) for income taxes, net	(91,810)	34,946	(179,321)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 139,616	$ 142,103	$ 94,288