CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
OPERATING ACTIVITIES
Net income	$ 111.6	$ 125.2
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	118.2	111.3
Deferred income taxes	1.1	(7.2)
Abandonment and impairment	0.3	0.1
Share-based compensation	5.4	6.3
Net (gain) from asset sales	(0.4)	(1.4)
(Income) from unconsolidated affiliate	(1.9)	(1.9)
Distributions from unconsolidated affiliate and other	2.8	2.6
Changes in operating assets and liabilities	207.8	66.2
NET CASH PROVIDED BY OPERATING ACTIVITIES	444.9	301.2
INVESTING ACTIVITIES
Property, plant and equipment purchased	(146.2)	(127.3)
Questar Gas acquisition of gas distribution system	0.0	(11.4)
Cash used in disposition of assets	(1.5)	(1.8)
Proceeds from disposition of assets	0.4	1.1
NET CASH USED IN INVESTING ACTIVITIES	(147.3)	(139.4)
FINANCING ACTIVITIES
Common stock issued	3.9	1.5
Common stock repurchased	(2.8)	(4.1)
Change in commercial paper, net	(248.6)	(119.0)
Short-term debt issued	250.0	0.0
Checks outstanding in excess of cash balances	4.0	2.5
Long-term debt repaid	(250.0)	0.0
Capital lease obligation repaid	(0.6)	(0.6)
Revolver issuance costs paid	(1.2)	0.0
Dividends paid	(77.5)	(74.1)
Tax benefits from share-based compensation	0.2	0.0
NET CASH USED IN FINANCING ACTIVITIES	(322.6)	(193.8)
Change in cash and cash equivalents	(25.0)	(32.0)
Beginning cash and cash equivalents	25.0	32.0
Ending cash and cash equivalents	0.0	0.0
Questar Gas [Member]
OPERATING ACTIVITIES
Net income	46.0	41.0
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	32.6	29.9
Deferred income taxes	12.5	(1.8)
Share-based compensation	0.8	0.7
Changes in operating assets and liabilities	182.3	95.0
NET CASH PROVIDED BY OPERATING ACTIVITIES	274.2	164.8
INVESTING ACTIVITIES
Property, plant and equipment purchased	(124.3)	(88.3)
Questar Gas acquisition of gas distribution system	0.0	(11.4)
Cash used in disposition of assets	(1.2)	(1.4)
Proceeds from disposition of assets	0.1	0.1
Affiliated-company property, plant and equipment transfers	(0.1)	(0.1)
NET CASH USED IN INVESTING ACTIVITIES	(125.5)	(101.1)
FINANCING ACTIVITIES
Change in notes payable to Questar	(144.3)	(59.6)
Checks outstanding in excess of cash balances	5.1	4.1
Dividends paid to Questar	(20.0)	(28.0)
NET CASH USED IN FINANCING ACTIVITIES	(159.2)	(83.5)
Change in cash and cash equivalents	(10.5)	(19.8)
Beginning cash and cash equivalents	10.5	19.8
Ending cash and cash equivalents	0.0	0.0
Questar Pipeline [Member]
OPERATING ACTIVITIES
Net income	27.8	29.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	28.6	28.9
Deferred income taxes	1.6	(1.1)
Share-based compensation	0.9	1.3
Net (gain) from asset sales	(0.4)	0.0
(Income) from unconsolidated affiliate	(1.9)	(1.9)
Distributions from unconsolidated affiliate and other	2.6	2.4
Changes in operating assets and liabilities	6.9	(5.9)
NET CASH PROVIDED BY OPERATING ACTIVITIES	66.1	52.8
INVESTING ACTIVITIES
Property, plant and equipment purchased	(14.2)	(17.1)
Cash used in disposition of assets	(0.3)	(0.4)
Proceeds from disposition of assets	0.3	0.1
Affiliated-company property, plant and equipment transfers	0.1	0.1
NET CASH USED IN INVESTING ACTIVITIES	(14.1)	(17.3)
FINANCING ACTIVITIES
Change in notes receivable from Questar	(28.6)	6.1
Dividends paid to Questar	(32.0)	(48.0)
NET CASH USED IN FINANCING ACTIVITIES	(60.6)	(41.9)
Change in cash and cash equivalents	(8.6)	(6.4)
Beginning cash and cash equivalents	10.2	7.4
Ending cash and cash equivalents	$ 1.6	$ 1.0