Condensed Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues [Abstract]
Natural gas sales	$ 41,757	$ 26,763	$ 96,881	$ 74,867
Natural gas transportation	422,999	389,080	1,265,285	1,116,891
Natural gas storage	33,839	33,954	102,392	102,778
Other	2,815	2,255	7,763	3,734
Total operating revenues	501,410	452,052	1,472,321	1,298,270
Operating Costs and Expenses:
Cost of natural gas sales	41,757	26,763	96,881	74,867
Cost of natural gas transportation	9,126	5,828	30,661	15,282
Operation and maintenance	101,916	113,101	282,591	267,914
Administrative and general	44,474	43,110	137,836	132,020
Depreciation and amortization	91,670	82,826	264,176	239,368
Taxes - other than income taxes	16,622	15,333	52,224	49,131
Regulatory credit resulting from Tax Reform (Note 1)	0	0	(20,867)	0
Other expense, net	15,631	13,475	46,036	43,112
Total operating costs and expenses	321,196	300,436	889,538	821,694
Operating Income	180,214	151,616	582,783	476,576
Other (Income) and Other Expenses:
Interest expense	50,180	41,304	148,629	115,797
Allowance for equity and borrowed funds used during construction (AFUDC)	(43,165)	(22,334)	(104,668)	(70,783)
Equity in earnings of unconsolidated affiliates	(762)	(912)	(497)	(3,322)
Miscellaneous other (income) expenses, net	(5,537)	(774)	(12,317)	(5,972)
Total other (income) and other expenses	716	17,284	31,147	35,720
Net Income	179,498	134,332	551,636	440,856
Other comprehensive income:
Equity interest in unrealized gain on interest rate hedges (includes $(51) and $38 for the three months ended and $(78) and $75 for the nine months ended September 30, 2018 and September 30, 2017, respectively, of accumulated other comprehensive income reclassification for equity interest in realized losses (gains) on interest rate hedges)	48	72	572	108
Comprehensive Income	$ 179,546	$ 134,404	$ 552,208	$ 440,964