CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net income	$ 466,802	$ 455,188	$ 453,383
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	416,427	383,992	365,713
Demand side management program amortization	3,509	4,331	4,802
Deferred income taxes	277,896	227,823	316,253
Amortization of Investment Tax Credits	(2,807)	(2,941)	(2,935)
Allowance for equity funds used during construction	(14,485)	(46,784)	(33,173)
Provision for bad debts	13,052	17,005	16,784
Net realized and unrealized hedging and derivative transactions	2,414	(2,578)	(3,571)
Other Operating Activities, Cash Flow Statement	2,500	0	0
Changes in operating assets and liabilities:
Accounts receivable	8,872	(42,921)	5,089
Accrued unbilled revenues	17,837	(23,132)	14,707
Inventories	33,417	(972)	(14,857)
Prepayments and other	10,483	(81,715)	(7,210)
Accounts payable	(40,982)	(22,789)	59,361
Net regulatory assets and liabilities	78,055	130,499	108,400
Other current liabilities	19,654	5,284	16,561
Pension and other employee benefit obligations	(23,449)	(38,905)	(48,886)
Change in other noncurrent assets	4,086	5,537	3,862
Change in other noncurrent liabilities	(35,334)	(19,130)	17,191
Net cash provided by (used in) operating activities	1,237,947	947,792	1,271,474
Investing activities
Utility capital/construction expenditures	(995,597)	(1,114,338)	(1,066,700)
Allowance for equity funds used during construction	14,485	46,784	33,173
Investments in utility money pool arrangement	(196,300)	(603,000)	(1,495,000)
Repayments from utility money pool arrangement	212,300	659,000	1,423,000
Net cash provided by (used in) investing activities	(965,112)	(1,011,554)	(1,105,527)
Financing activities
Proceeds from (repayments of) short-term borrowings, net	(368,000)	382,000	(154,000)
Borrowings under utility money pool arrangement	165,000	333,000	14,000
Repayments under utility money pool arrangement	(165,000)	(333,000)	(14,000)
Proceeds from issuance of long-term debt	246,751	295,598	492,313
Repayments of long-term debt	0	(275,000)	(250,000)
Capital contributions from parent	175,210	81,498	25,621
Dividends paid to parent	(330,846)	(433,788)	(263,942)
Net cash provided by (used in) financing activities	(276,885)	50,308	(150,008)
Net change in cash and cash equivalents	(4,050)	(13,454)	15,939
Cash and cash equivalents at beginning of period	7,635	21,089	5,150
Cash and cash equivalents at end of period	3,585	7,635	21,089
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(165,546)	(150,011)	(155,457)
Cash received (paid) for income taxes, net	13,822	(91,810)	34,946
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 106,912	$ 139,616	$ 142,103