Capital Stock (Narrative) (Details) - USD ($)		12 Months Ended
	Oct. 05, 2016	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Oct. 31, 2017	Apr. 30, 2017
Class of Stock [Line Items]
Authorized amount of share repurchase program		$ 750,000,000				$ 300,000,000
Common Stock, Par or Stated Value Per Share		$ 0.01		$ 0.01
Incremental increase to authorized amount of share repurchase program			$ 250,000,000		$ 200,000,000
Warrants exercised (in shares)		204
Warrants issued (in shares)		1,845,954
Common Class A [Member]
Class of Stock [Line Items]
Common Stock, Par or Stated Value Per Share	$ 0.01
Investment Warrants, Exercise Price	$ 57.00
Warrants issued (in shares)	1,914,856