Derivatives and Hedging (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2016	Dec. 31, 2011	Dec. 31, 2015	Jun. 30, 2015
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net		$ (1.3)		$ (0.2)	$ (0.3)
Cash Flow Hedging [Member] | Questar Pipeline [Member] | Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]
Derivative [Line Items]
Notional amount	$ 150.0		$ 150.0
Debt Instrument, Face Amount	180.0		$ 180.0
Interest payment for terminated derivative instrument	$ 37.3
Life of long-term debt associated with cash flow hedges			30 years
Date Through Which Reclassifications Into Earnings From AOCI Will Take Place		2041
Pre-tax net losses expected to be reclassified from AOCI to Consolidated statement of income		$ 0.6
Reclassification from accumulated OCI to income, time to transfer		12 months