CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Current Assets
Cash and cash equivalents	$ 7.2	$ 16.8	$ 8.1
Accounts receivable, net	141.2	114.3	129.8
Unbilled gas accounts receivable	50.5	78.3	42.7
Gas stored underground	9.8	39.0	13.0
Materials and supplies	30.0	24.5	31.9
Current regulatory assets	31.3	46.7	26.1
Prepaid expenses and other	6.8	13.1	6.4
Deferred income taxes - current	17.2	13.0	16.2
Total Current Assets	294.0	345.7	274.2
Property, Plant and Equipment	5,399.4	5,333.3	5,101.7
Accumulated depreciation, depletion and amortization	(2,059.1)	(2,016.3)	(1,926.6)
Net Property, Plant and Equipment	3,340.3	3,317.0	3,175.1
Investment in unconsolidated affiliate	26.1	26.5	27.0
Noncurrent regulatory assets	20.4	21.6	20.1
Other noncurrent assets	47.8	46.3	43.4
TOTAL ASSETS	3,728.6	3,757.1	3,539.8
Current Liabilities
Short-term debt	202.0	263.0	129.0
Accounts payable and accrued expenses	222.8	235.2	180.4
Current regulatory liabilities	36.8	5.8	62.4
Current portion of long-term debt and capital lease obligation	2.8	42.7	132.4
Total Current Liabilities	464.4	546.7	504.2
Long-term debt and capital lease obligation, less current portion	1,137.6	1,138.2	992.2
Deferred income taxes	633.1	603.4	540.0
Asset retirement obligations	61.2	67.2	64.1
Defined benefit pension plan and other postretirement benefits	279.2	303.7	290.4
Customer contributions in aid of construction	23.8	22.9	26.2
Regulatory and other noncurrent liabilities	48.1	39.4	41.7
COMMON SHAREHOLDERS' EQUITY
Common stock	444.8	442.4	516.0
Retained earnings	882.6	839.5	791.1
Accumulated other comprehensive (loss)	(246.2)	(246.3)	(226.1)
Total Common Shareholders' Equity	1,081.2	1,035.6	1,081.0
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	3,728.6	3,757.1	3,539.8
Questar Gas [Member]
Current Assets
Cash and cash equivalents	2.8	1.4	4.5
Accounts receivable, net	103.7	67.0	86.1
Unbilled gas accounts receivable	50.4	78.2	42.6
Accounts receivable from affiliates	32.8	31.8	11.6
Gas stored underground	9.6	38.3	12.9
Materials and supplies	14.4	13.5	13.9
Current regulatory assets	26.1	41.0	20.8
Prepaid expenses and other	2.2	3.2	1.8
Deferred income taxes - current	5.7	1.5	5.6
Total Current Assets	247.7	275.9	199.8
Property, Plant and Equipment	2,062.8	2,040.9	1,958.5
Accumulated depreciation, depletion and amortization	(772.1)	(762.8)	(760.0)
Net Property, Plant and Equipment	1,290.7	1,278.1	1,198.5
Noncurrent regulatory assets	15.7	16.4	14.7
Goodwill	5.6	5.6	5.6
Other noncurrent assets	3.1	3.1	2.4
TOTAL ASSETS	1,562.8	1,579.1	1,421.0
Current Liabilities
Notes payable to Questar	132.3	166.1	66.4
Accounts payable and accrued expenses	109.9	105.0	105.6
Accounts payable to affiliates	51.1	50.9	45.4
Customer advances	7.2	30.2	6.7
Current regulatory liabilities	35.6	4.3	60.9
Current portion of long-term debt and capital lease obligation	2.0	42.0	131.5
Total Current Liabilities	338.1	398.5	416.5
Long-term debt and capital lease obligation, less current portion	384.5	384.5	236.5
Deferred income taxes	316.5	301.6	257.8
Customer contributions in aid of construction	23.8	22.9	26.2
Regulatory and other noncurrent liabilities	3.8	4.0	4.7
COMMON SHAREHOLDERS' EQUITY
Common stock	23.0	23.0	23.0
Additional paid-in capital	172.9	172.5	171.7
Retained earnings	300.2	272.1	284.6
Total Common Shareholders' Equity	496.1	467.6	479.3
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	1,562.8	1,579.1	1,421.0
Questar Pipeline [Member]
Current Assets
Cash and cash equivalents	5.4	7.1	4.6
Notes receivable from Questar	33.1	38.7	14.5
Accounts receivable, net	18.6	19.1	19.3
Accounts receivable from affiliates	34.0	27.5	23.8
Materials and supplies	10.9	7.2	8.9
Current regulatory assets	5.2	5.7	5.3
Prepaid expenses and other	3.3	7.7	3.1
Deferred income taxes - current	1.6	1.6	1.8
Total Current Assets	112.1	114.6	81.3
Property, Plant and Equipment	1,852.7	1,840.1	1,797.6
Accumulated depreciation, depletion and amortization	(642.7)	(629.3)	(599.6)
Net Property, Plant and Equipment	1,210.0	1,210.8	1,198.0
Investment in unconsolidated affiliate	26.1	26.5	27.0
Goodwill	4.2	4.2	4.2
Regulatory and other noncurrent assets	8.7	9.0	9.5
TOTAL ASSETS	1,361.1	1,365.1	1,320.0
Current Liabilities
Accounts payable and accrued expenses	31.5	25.7	28.0
Accounts payable to affiliates	3.4	16.9	4.0
Current regulatory liabilities	1.2	1.5	1.5
Total Current Liabilities	36.1	44.1	33.5
Long-term debt and capital lease obligation, less current portion	458.9	458.9	458.9
Deferred income taxes	245.8	242.5	210.6
Regulatory and other noncurrent liabilities	14.0	13.8	13.5
COMMON SHAREHOLDERS' EQUITY
Common stock	6.6	6.6	6.6
Additional paid-in capital	348.1	347.5	346.2
Retained earnings	274.7	274.9	274.1
Accumulated other comprehensive (loss)	(23.1)	(23.2)	(23.4)
Total Common Shareholders' Equity	606.3	605.8	603.5
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	$ 1,361.1	$ 1,365.1	$ 1,320.0