Derivative Instruments, Hedging Activities and Fair Value (Details 6) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative Instruments, Hedging Activities And Fair Value Disclosure [Abstract]
Net derivative asset subject to master netting arrangements		$ 0
Long-term debt, including current maturities
Fair value of long-term debt	$ 946,500	885,000
Carrying value of long-term debt	$ 930,800	$ 854,900