CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Net income $ 84.6 $ 85.1 $ 226.0 $ 173.4
Other comprehensive income (loss):        
Pension and other postretirement benefits 6.0 4.3 (95.2) 165.3
Interest rate cash flow hedge amortization 0.2 0.2 0.5 0.5
Change in fair value of long-term investment 0 0 0 (0.1)
Income taxes (2.4) (1.8) 36.3 (63.5)
Net other comprehensive income (loss) 3.8 2.7 (58.4) 102.2
COMPREHENSIVE INCOME 88.4 87.8 167.6 275.6
Questar Pipeline [Member]        
Net income 13.9 15.7 58.8 8.1
Other comprehensive income (loss):        
Interest rate cash flow hedge amortization 0.2 0.2 0.5 0.5
Income taxes (0.1) (0.1) (0.1) (0.2)
Net other comprehensive income (loss) 0.1 0.1 0.4 0.3
COMPREHENSIVE INCOME $ 14.0 $ 15.8 $ 59.2 $ 8.4