Stock-Based Compensation and Other Employee Benefit Plans (Schedule of Activity, Employee Stock Purchase Plans) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock [Member]
Stock-Based Compensation and Other Employee Benefit Plans [Line Items]
Common shares issued (in shares)	92,145	144,183	134,389
Employee Stock Purchase Plans [Member]
Stock-Based Compensation and Other Employee Benefit Plans [Line Items]
Cash proceeds (in dollars)	$ 3.3	$ 2.7	$ 2.0
Employee Stock Purchase Plans [Member] | Common Stock [Member]
Stock-Based Compensation and Other Employee Benefit Plans [Line Items]
Weighted average price per share (in dollars per share)	$ 35.92	$ 18.55	$ 14.67
Weighted average per share grant date fair value (in dollars per share)	$ 9.24	$ 5.00	$ 4.22