Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Tax expense recognized on the amortization of prior service cost	$ (27)	$ (24)	$ (24)
Tax expense recognized on the net gain (loss)	73	(1,997)	1,673
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	$ (150)	$ (22)	$ 0