Derivative Financial Instruments (Details) (Interest rate contract, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
HBB
Derivative [Line Items]
Notional amount of interest rate derivatives	$ 20.0	$ 20.0
Average fixed interest rate of interest rate derivatives	1.40%	1.40%
Remaining term	extending to January 2020
NACoal
Derivative [Line Items]
Notional amount of interest rate derivatives	$ 100.0	$ 100.0
Average fixed interest rate of interest rate derivatives	1.40%	1.40%
Remaining term	extending to May 2018