Equity (Share Activity) (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2016
Equity, Treasury Stock [Line Items]
Treasury stock acquired, value	$ 70,100,000			$ 198,400,000
2016 Share Repurchase Program [Member]
Equity, Treasury Stock [Line Items]
Treasury stock acquired, value		$ 98,400,000
Share repurchase program, amount authorized, value					$ 100,000,000
March 2018 Share Repurchase Program [Member]
Equity, Treasury Stock [Line Items]
Share repurchase program, amount authorized, value		$ 100,000,000
September 2018 Share Repurchase Program [Member]
Equity, Treasury Stock [Line Items]
Share repurchase program, amount authorized, value	150,000,000		$ 150,000,000	150,000,000
Share repurchase program, amount available for future share repurchases, value	$ 150,000,000		$ 150,000,000	$ 150,000,000
Treasury Stock, Common [Member]
Equity, Treasury Stock [Line Items]
Treasury stock acquired (in shares)	1.6			4.3
Treasury Stock, Common [Member] | 2016 Share Repurchase Program [Member]
Equity, Treasury Stock [Line Items]
Treasury stock acquired (in shares)		2.0
Treasury Stock, Common [Member] | March 2018 Share Repurchase Program [Member]
Equity, Treasury Stock [Line Items]
Treasury stock acquired (in shares)			2.3