Long-term debt (Details) (USD $) In Millions, unless otherwise specified						0 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	May 08, 2014 The Company [Member]	Apr. 15, 2014 The Company [Member]	Jan. 28, 2014 The Company [Member]	Apr. 02, 2014 Centennial Energy Holdings, Inc [Member]	Mar. 31, 2014 Centennial Energy Holdings, Inc [Member]	May 08, 2014 Centennial Energy Holdings, Inc [Member]	Jul. 15, 2014 Subsequent event [Member] The Company [Member]
Line of Credit Facility [Line Items]
Note purchase agreement					$ 150.0
Senior notes issued				50.0					100.0
Senior notes weighted average interest rate				5.20%					4.30%
Facility, maximum borrowing capacity			175.0					650.0
Ratio of total debt to total capitalization as specified in debt convenants						65.00%	65.00%	65.00%
Debt instrument, term						2 years	2 years
Term loan agreement						125.0	125.0
Long-term commercial paper	$ 268.0	$ 153.9