Shareholders' Equity Common Stock Repurchase Plans (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended		12 Months Ended
	Nov. 30, 2016	Apr. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Common stock repurchase amount				$ 18,545,000	$ 50,000,000
Common Stock | April 2015 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Treasury stock, value		$ 50,000,000.0
Common stock repurchase amount				$ 50,000,000.0
Average cost per share of treasury stock acquired (usd per share)				$ 12.57
Acquisition of treasury stock (in shares)				(4.0)
Percent of common shares outstanding, expected to be delivered		2.30%
Common Stock | November 2016 Stock Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Treasury stock, value	$ 50,000,000.0
Common stock repurchase amount			$ 18,500,000.0
Average cost per share of treasury stock acquired (usd per share)			$ 12.48
Remaining repurchase amount			$ 31,500,000.0
Acquisition of treasury stock (in shares)			(1.5)
Percent of common shares outstanding, expected to be delivered	2.30%