Shareholders' Equity - Schedule of Stock Repurchases (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Repurchase shares authorized (in shares)	5,000,000
Remaining number of shares authorized to be repurchased (in shares)	3,100,000
Total shares repurchased (in shares)	1,473,054	2,038,368	2,449,159
Purchase price	$ 68,896	$ 69,091	$ 96,169
Average price of shares repurchased (in dollars per share)	$ 46.77	$ 33.90	$ 39.27