Per Share Data - Narrative (Details) - $ / shares	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Aug. 27, 2015
Earnings Per Share [Abstract]
Convertible securities which would affect denominator in calculating basic and dilutive earnings per share (in shares)	0
Options, outstanding (in shares)	423,700	263,700	118,500	93,500	23,500
Outstanding, weighted average exercise price (in dollars per share)	$ 43.94	$ 45.12	$ 43.91	$ 43.59
Share price (in dollars per share)	$ 40.10		$ 40.36