Shareholders' Equity - Treasury Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended		14 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Nov. 14, 2016	May 02, 2012
Class of Stock [Line Items]
Purchases (in shares)	27,950	24,086	100,000
Purchase of treasury stock (less than)	$ 0.1	$ 0.2	$ 0.3
Remaining number of shares authorized to be repurchased (less than) (in shares)	500,000		500,000
Average price per share of treasury stock acquired and held (dollars per share)	$ 41.93
May 2, 2012 Share Repurchase Program
Class of Stock [Line Items]
Number of shares authorized to be repurchased (in shares)					500,000.0
November 14, 2016 Share Repurchase Program
Class of Stock [Line Items]
Number of shares authorized to be repurchased (in shares)				500,000.0