Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2019	Feb. 16, 2018	Jan. 12, 2017	Jun. 04, 2015	May 20, 2010	Dec. 31, 2007
Class of Stock [Line Items]
Shares registered	400,000,000	400,000,000
Common stock, shares issued	112,433,000	116,446,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased, shares		5,800,000
Shares registered									4,000,000
Shares reclassified as authorized and unissued	1,200,000
Stock Repurchase Program One [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized							$ 1,000.0
Stock Repurchase Program Two [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized							$ 1,000.0
Stock repurchased, shares		42,163	5,300,000
Stock repurchased, value		$ 4.4	$ 545.7
Weighted average price per share		$ 104.81	$ 102.14
Stock Repurchase Program Three [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized						$ 1,000.0
Stock repurchased, value		$ 850.0
Weighted average price per share		$ 146.17
Stock Repurchase Program Four [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized					$ 1.0
Stock repurchased, shares	5,000,000
Stock repurchased, value	$ 795.5
Weighted average price per share	$ 159.87
Stock Repurchase Program Four [Member] | Class A Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization				$ 204.5
2010 Plan [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Maximum issuance of shares	15,000,000							15,000,000
Common stock, shares issued	0	487,963	0