Debt Short-term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015
Short-term Debt [Line Items]
Commercial paper with various interest rates	$ 347.0	$ 457.6
Revolving credit arrangement [Member]
Short-term Debt [Line Items]
Revolving credit facility credit commitments	$ 750.0	500.0
Line of Credit Facility, Maximum Borrowing Capacity 364-day		$ 250.0
Revolving credit facility, maturity date	Apr. 19, 2018
Consolidated funded debt percentage of consolidated capital permitted under credit facility amendment		70.00%
Revolving credit facility, remaining borrowing capacity		$ 292.4
Commercial paper [Member]
Short-term Debt [Line Items]
Commercial paper with various interest rates	$ 347.0	$ 457.6
Weighted-average interest rate at end of year	0.23%	0.50%
Questar Gas [Member]
Short-term Debt [Line Items]
Notes payable to parent	$ 119.3	$ 273.3
Questar Gas [Member] | Notes payable to Questar [Member]
Short-term Debt [Line Items]
Weighted-average interest rate at end of year	0.25%	0.35%
Notes payable to parent	$ 119.3	$ 273.3
Questar Pipeline [Member] | Notes payable to Questar [Member]
Short-term Debt [Line Items]
Notes payable to parent	$ 0.0	$ 0.0