Stock Repurchase Program (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2017	Feb. 29, 2004
Stock repurchase program
Stock Repurchased During Period, Shares			60,000
Stock Repurchased During Period, Value			$ 4.3
Announced stock repurchase program
Stock repurchase program
Shares authorized under repurchase program					250,000
Stock Repurchased During Period, Shares	0	32,900
Stock Repurchased During Period, Value		$ 2.1
Other stock repurchase arrangements
Stock repurchase program
Stock Repurchased During Period, Shares	61,125	55,537	21,046
Stock Repurchased During Period, Value	$ 5.5	$ 3.9	$ 1.4
2004 plan
Stock repurchase program
Shares authorized under repurchase program				1,000,000