Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Tax expense recognized on the amortization of prior service cost	$ (31,000)	$ (29,000)	$ (27,000)
Tax expense recognized on the net gain (loss)	432,000	178,000	73,000
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	$ (8,000)	$ 496,000	$ (150,000)