Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operating activities:
Net income	$ 374,026	$ 272,466	$ 279,658
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	263,949	266,981	260,069
Allowance for equity funds used during construction (equity AFUDC)	(13,299)	(13,222)	(10,588)
Changes in operating assets and liabilities:
Receivables - affiliates	55	3,247	(982)
Receivables - trade and other	(21,772)	(4,186)	(11,155)
Transportation and exchange gas receivable	(3,881)	2,038	(2,497)
Regulatory assets - current	28,536	1,171	10,567
Regulatory assets - non-current	21,910	(5,980)	(8,698)
Inventories	232	673	50,295
Payables - affiliates	(3,738)	15,069	(1,832)
Payables - trade	(26,463)	3,727	14,577
Accrued liabilities	28,322	8,256	27,775
Asset retirement obligations	13,105	35,195	(6,964)
Asset retirement obligation - removal costs	(26,919)	(41,052)	(43,666)
Reserve for rate refunds	98,217	0	0
Other, net	10,731	(2,934)	15,085
Net cash provided by operating activities	743,011	541,449	571,644
Cash flows from financing activities:
Additions to long-term debt	0	398,804	372,518
Retirement of long-term debt	0	(325,000)	(300,000)
Debt issue costs	0	(4,403)	(3,846)
Cash distributions to parent	(250,000)	(246,259)	(219,000)
Cash contributions from parent	264,000	150,000	115,000
Other, net	(3,034)	(3,333)	(4,681)
Net cash provided by (used in) financing activities	10,966	(30,191)	(40,009)
Cash flows from investing activities:
Property, plant and equipment additions, net of equity AFUDC	(526,916)	(475,450)	(385,671)
Disposal of property, plant and equipment, net	(3,621)	7,157	2,698
Advances to affiliate, net	(214,215)	(58,554)	(144,773)
Return of capital from unconsolidated affiliates	1,438	11,327	1,925
Contributions to unconsolidated affiliates	0	(5,806)	(14,834)
Purchase of ARO Trust investments	(58,242)	(34,430)	(41,310)
Proceeds from sale of ARO Trust investments	45,607	43,205	56,576
Other, net	1,900	1,314	(6,230)
Net cash used in investing activities	(754,049)	(511,237)	(531,619)
Increase (decrease) in cash	(72)	21	16
Cash at beginning of period	185	164	148
Cash at end of period	113	185	164
Increase to property, plant and equipment	(558,201)	(466,115)	(386,462)
Changes in related accounts payable and accrued liabilities	31,285	(9,335)	791
Property, plant and equipment additions, net of equity AFUDC	(526,916)	(475,450)	(385,671)
Supplemental Cash Flow Elements [Abstract]
Interest (exclusive of amount capitalized)	76,803	86,586	88,357
Income taxes	$ 116	$ 254	$ 728