Derivatives and Hedging (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2015	Mar. 31, 2015
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (0.4)			$ (0.2)	$ 0.0
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	$ 0.5
Reclassification from accumulated OCI to income, time to transfer	12 months