Share Repurchases (Details) - USD ($) $ / shares in Units, shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 25, 2017
Equity, Class of Treasury Stock [Line Items]
Payments under share repurchase program	$ 25,000,000	$ 0	$ 39,053,000
Share Repurchase Plan 2017 [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares authorized to be repurchased				$ 200,000,000.0
Number of shares repurchased	0.3
Payments under share repurchase program	$ 25,000,000
Average price per share of shares repurchased (in dollars per share)	$ 79.75