(Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Oct. 31, 2017	Apr. 30, 2017
Equity [Abstract]
Authorized amount of share repurchase program	$ 500		$ 300
Incremental increase to authorized amount of share repurchase program		$ 200
Document Fiscal Year Focus	2017
Warrants exercised (in shares)	65,499
Warrants issued (in shares)	1,846,158