Shareholders' Equity (Narrative) (Details) - USD ($)	1 Months Ended	3 Months Ended	6 Months Ended
	Jul. 31, 2017	Jun. 30, 2017	Jun. 30, 2017	Aug. 04, 2017	Jan. 12, 2017	Dec. 31, 2016	Jun. 04, 2015	Dec. 31, 2007
Class of Stock [Line Items]
Stock repurchase program, authorized							$ 1,000,000,000
Stock repurchase program, remaining authorization					$ 150,000,000
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization				$ 750,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased, shares		1,200,000	1,200,000
Stock repurchased, value		$ 155,000,000	$ 155,000,000
Weighted average price per share		$ 134.41	$ 134.41
Class A Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchased, shares	700,000
Stock repurchased, value	$ 95,000,000
Weighted average price per share	$ 135.92
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