Consolidated Statements of Comprehensive Income (Parenthetical) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Tax expense recognized on the amortization of prior service cost	$ (24)	$ (24)	$ (26)
Tax expense recognized on the net gain (loss)	(1,997)	1,673	(331)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	$ (33)	$ 0	$ 0