Consolidated Statements of Income and Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Accumulated other comprehensive income reclassification for change in fair value of interest rate swap agreements	$ 288	$ 340	$ 378
Accumulated other comprehensive income reclassifications for net realized gains on available-for-sale securities	1,543	1,005	666
Accumulated other comprehensive income reclassifications for net amortization of actuarial losses	192	184	173
Income tax expense from reclassification	$ 372	$ 168	$ 40