Derivative instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Derivative instruments [Line Items]
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI		$ 1,800,000
Amount of time remaining to transfer accumulated other comprehensive income to earnings	12 months
Cash flow hedge gain (loss) to be reclassified within twelve months	(700,000)
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	7,500,000		6,300,000
Assets needed for immediate settlement aggregate fair value for cash flow heding instruments	7,500,000		6,300,000
Oil swap and or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	2,900,000		2,600,000
Natural gas swap or and collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	18,300,000		11,000,000
Interest rate derivative instruments
Derivative instruments [Line Items]
Notional amount of interest rate cash flow hedge derivatives			$ 50,000,000