Derivatives (Details 5) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Open interest rate contracts
Initial Notional Amount (in millions)	$ 63.0
Not designated as hedging | Long-term Interest Rate Swap, 2012
Open interest rate contracts
Year Entered	2012
Year of Maturity	2023
Initial Notional Amount (in millions)	$ 23.0
Hedged Item	Interest rate component of debt - not accounted for as a hedge
Interest Rate	1.90%
Not designated as hedging | Long-term Interest Rate Collar, 2013 | Cash flow hedge
Open interest rate contracts
Year Entered	2013
Year of Maturity	2021
Initial Notional Amount (in millions)	$ 40.0
Hedged Item	Interest rate component of debt - not accounted for as a hedge
Not designated as hedging | Long-term Interest Rate Collar, 2013 | Cash flow hedge | Minimum
Open interest rate contracts
Interest Rate	2.90%
Not designated as hedging | Long-term Interest Rate Collar, 2013 | Cash flow hedge | Maximum
Open interest rate contracts
Interest Rate	4.80%