Shareholders' Equity - Share Repurchases (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Shareholders Equity [Abstract]
Total number of shares repurchased and retired	1,028	2,908	2,179
Total investment	$ 90,461	$ 245,816	$ 293,616