Fair Value of Financial Instruments - Summary of fair value of financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Short-Term Debt	$ (37,173)	$ (80,672)
Long-Term Debt including Current Maturities	(546,247)	(496,268)
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Short-Term Debt	(37,173)	(80,672)
Long-Term Debt including Current Maturities	$ (618,875)	$ (561,245)