CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Questar Gas [Member]	Questar Gas [Member] Common Stock [Member]	Questar Gas [Member] Additional Paid-in Capital [Member]	Questar Gas [Member] Retained Earnings [Member]	Questar Pipeline [Member]	Questar Pipeline [Member] Common Stock [Member]	Questar Pipeline [Member] Additional Paid-in Capital [Member]	Questar Pipeline [Member] Retained Earnings [Member]	Questar Pipeline [Member] Accumulated Other Comprehensive Loss [Member]
Beginning balances at Dec. 31, 2012	$ 1,035.6	$ 442.4	$ 839.5	$ (246.3)	$ 467.6	$ 23.0	$ 172.5	$ 272.1	$ 605.8	$ 6.6	$ 347.5	$ 274.9	$ (23.2)
Beginning balances (in shares) at Dec. 31, 2012		175.0
Equity contribution from Questar					90.0	0.0	90.0	0.0
Common stock issued	2.3	$ 2.3	0.0	0.0
Common stock issued (in shares)		0.3
Common stock repurchased	(4.2)	$ (4.2)	0.0	0.0
Common stock repurchased (in shares)		(0.2)
Dividends paid	(124.6)	$ 0.0	(124.6)	0.0	(35.5)	0.0	0.0	(35.5)	(64.0)	0.0	0.0	(64.0)	0.0
Share-based compensation	10.0	10.0	0.0	0.0	1.4	0.0	1.4	0.0	2.0	0.0	2.0	0.0	0.0
Tax benefits from share-based compensation	14.0	14.0	0.0	0.0
Net income	161.2	0.0	161.2	0.0	52.8	0.0	0.0	52.8	8.2	0.0	0.0	8.2	0.0
Net other comprehensive income (loss)	104.5	0.0	0.0	104.5					0.3	0.0	0.0	0.0	0.3
Ending balances at Dec. 31, 2013	1,198.8	$ 464.5	876.1	(141.8)	576.3	23.0	263.9	289.4	552.3	6.6	349.5	219.1	(22.9)
Ending balances (in shares) at Dec. 31, 2013		175.1
Common stock issued	2.9	$ 2.9	0.0	0.0
Common stock issued (in shares)		0.5
Common stock repurchased	(4.1)	$ (4.1)	0.0	0.0
Common stock repurchased (in shares)		(0.2)
Dividends paid	(131.9)	$ 0.0	(131.9)	0.0	(36.0)	0.0	0.0	(36.0)	(64.0)	0.0	0.0	(64.0)	0.0
Share-based compensation	11.3	11.3	0.0	0.0	1.5	0.0	1.5	0.0	1.9	0.0	1.9	0.0	0.0
Tax benefits from share-based compensation	2.2	2.2	0.0	0.0
Net income	226.5	0.0	226.5	0.0	55.2	0.0	0.0	55.2	60.6	0.0	0.0	60.6	0.0
Net other comprehensive income (loss)	(59.5)	0.0	0.0	(59.5)					0.4	0.0	0.0	0.0	0.4
Ending balances at Dec. 31, 2014	1,246.2	$ 476.8	970.7	(201.3)	597.0	23.0	265.4	308.6	551.2	6.6	351.4	215.7	(22.5)
Ending balances (in shares) at Dec. 31, 2014		175.4
Common stock issued	3.5	$ 3.5	0.0	0.0
Common stock issued (in shares)		0.8
Common stock repurchased	(23.3)	$ (23.3)	0.0	0.0
Common stock repurchased (in shares)		(1.2)
Dividends paid	(148.0)	$ 0.0	(148.0)	0.0	(38.0)	0.0	0.0	(38.0)	(64.0)	0.0	0.0	(64.0)	0.0
Share-based compensation	11.4	11.4	0.0	0.0	1.4	0.0	1.4	0.0	2.0	0.0	2.0	0.0	0.0
Tax benefits from share-based compensation	0.6	0.6	0.0	0.0
Net income	208.7	0.0	208.7	0.0	64.3	0.0	0.0	64.3	59.6	0.0	0.0	59.6	0.0
Net other comprehensive income (loss)	16.0	0.0	0.0	16.0					0.2	0.0	0.0	0.0	0.2
Ending balances at Dec. 31, 2015	$ 1,315.1	$ 469.0	$ 1,031.4	$ (185.3)	$ 624.7	$ 23.0	$ 266.8	$ 334.9	$ 549.0	$ 6.6	$ 353.4	$ 211.3	$ (22.3)
Ending balances (in shares) at Dec. 31, 2015		175.0