Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
OPERATING ACTIVITIES:
Net income	$ 178,696	$ 170,553	$ 172,987
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation	100,554	99,138	96,109
Regulatory debits	2,550	1,487	921
Gain on sale of property, plant and equipment	0	(70)	0
Amortization of deferred charges and credits	762	894	1,720
Allowance for equity funds used during construction	(1,099)	(391)	(945)
Changes in current assets and liabilities:
Trade and other accounts receivable	(137)	395	2,133
Affiliated receivables	598	38,388	(38,744)
Exchange gas due from others	7,349	(1,433)	153
Materials and supplies	(117)	203	(132)
Other current assets	(1,283)	(1,326)	(202)
Trade accounts payable	158	(4,958)	(212)
Affiliated payables	(1,184)	2,007	224
Exchange gas due to others	(8,095)	2,179	(153)
Other accrued liabilities	5,410	(3,095)	3,885
Changes in noncurrent assets and liabilities:
Deferred charges	(2,633)	(2,087)	(7,448)
Noncurrent liabilities	9,164	11,000	9,819
Net cash provided by operating activities	290,693	312,884	240,115
FINANCING ACTIVITIES:
Capital contributions from parent	0	0	13,300
Cash distributions to parent	(168,000)	(234,000)	(91,000)
Other	0	(622)	(1,318)
Net cash used in financing activities	(168,000)	(234,622)	(79,018)
Property, plant and equipment:
Capital expenditures, net of equity AFUDC	(86,289)	(79,413)	(83,317)
Contributions and Advances for Construction Costs	1,499	3,056	6,326
Disposal of property, plant, and equipment, net	(1,075)	5,728	5,756
Advances to affiliates, net	(36,813)	(15,886)	(89,846)
Proceeds from insurance	0	8,274	0
Net cash used in investing activities	(122,678)	(78,241)	(161,081)
NET INCREASE IN CASH	15	21	16
CASH AT BEGINNING OF PERIOD	154	133	117
CASH AT END OF PERIOD	169	154	133
Supplemental Cash Flow Elements [Abstract]
Increases to property, plant and equipment	(73,931)	(71,207)	(88,123)
Changes in accounts receivable, accounts payable, and accrued liabilities	(12,358)	(8,206)	4,806
Capital expenditures, net of equity AFUDC	$ (86,289)	$ (79,413)	$ (83,317)