Shareholders' Equity - Stock Repurchase Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Shares Repurchased (in shares)	108,109	459,264	1,058,018
Weighted- Average Price per Share (in dollars per share)	$ 143.15	$ 126.44	$ 124.01
Dollar Amount Repurchased	$ 15,475	$ 58,069	$ 127,443