CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net income	$ 1,123,379	$ 984,485	$ 1,021,306
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	1,318,752	1,142,966	1,036,515
Conservation and demand side management program amortization	3,854	5,225	6,033
Nuclear fuel amortization	116,982	106,424	114,542
Deferred income taxes	586,650	535,868	569,378
Amortization of investment tax credits	(5,203)	(5,277)	(5,543)
Allowance for equity funds used during construction	(60,547)	(55,936)	(89,750)
Equity earnings of unconsolidated subsidiaries	(42,123)	(34,390)	(30,151)
Dividends from unconsolidated subsidiaries	46,170	40,128	36,707
Provision for bad debts	38,960	36,074	42,765
Share-based compensation expense	41,170	44,928	32,189
Loss on Monticello life cycle management/extended power uprate project	0	129,463	0
Net realized and unrealized hedging and derivative transactions	7,939	21,919	5,506
Other, net	(889)	(1,326)	0
Changes in operating assets and liabilities:
Accounts receivable	(83,170)	65,826	(125,146)
Accrued unbilled revenues	(74,965)	73,625	(41,262)
Inventories	1,349	(11,240)	(20,558)
Other current assets	61,515	9,273	(111,300)
Accounts payable	117,744	(120,002)	(53,242)
Net regulatory assets and liabilities	(19,378)	102,465	195,823
Other current liabilities	20,249	78,158	148,441
Pension and other employee benefit obligations	(90,707)	(69,256)	(101,457)
Change in other noncurrent assets	(16,191)	10,553	44,364
Change in other noncurrent liabilities	(39,241)	(52,090)	(15,674)
Net cash provided by operating activities	3,052,299	3,037,863	2,659,486
Investing activities
Utility capital/construction expenditures	(3,255,550)	(3,683,359)	(3,199,791)
Allowance for equity funds used during construction	60,547	55,936	89,750
Proceeds from insurance recoveries	4,509	27,237	6,000
Purchases of investment securities	(546,612)	(1,257,924)	(595,569)
Proceeds from the sale of investment securities	478,866	1,236,873	588,430
Investments in WYCO Development LLC and other	(3,962)	(1,392)	(2,376)
Other, net	789	(145)	(3,695)
Net cash used in investing activities	(3,261,413)	(3,622,774)	(3,117,251)
Financing activities
(Repayments of) proceeds from short-term borrowings, net	(454,000)	(173,500)	260,500
Proceeds from issuance of long-term debt	2,423,768	1,626,212	837,584
Repayments of long-term debt	(1,035,901)	(250,882)	(275,948)
Proceeds from issuance of common stock	0	7,011	180,798
Repurchases of common stock	(32,209)	0	0
Dividends paid	(680,521)	(606,574)	(561,411)
Other	(12,487)	(12,024)	(11,294)
Net cash provided by financing activities	208,650	590,243	430,229
Net change in cash and cash equivalents	(464)	5,332	(27,536)
Cash and cash equivalents at beginning of period	84,940	79,608	107,144
Cash and cash equivalents at end of period	84,476	84,940	79,608
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(591,996)	(542,860)	(512,602)
Cash received (paid) for income taxes, net	61,933	58,287	(4,542)
Supplemental disclosure of non-cash investing and financing transactions:
Property, plant and equipment additions in accounts payable	253,955	321,969	417,473
Issuance of common stock for reinvested dividends and equity awards	$ 29,427	$ 52,911	$ 62,078