Shareholders' Equity (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017	Sep. 30, 2017	Jan. 12, 2017	Dec. 31, 2016	Jun. 04, 2015	Dec. 31, 2007
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization	$ 350,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased, shares		2,700,000	3,900,000
Stock repurchased, value		$ 383,900,000	$ 538,900,000
Weighted average price per share		$ 141.17	$ 139.16
Class A Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchased, shares	800,000
Stock repurchased, value	$ 111,100,000
Weighted average price per share	$ 147.19
Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized		$ 1,000,000,000	$ 1,000,000,000
Previous Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized						$ 1,000,000,000
Stock repurchase program, remaining authorization				$ 150,000,000
2007 [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Shares registered							4,000,000
Common stock, shares issued		487,963	487,963		0
Shares reclassified as authorized and unissued			1,200,000