Shareholders' Equity (Narrative) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 12, 2017	Jun. 04, 2015	Apr. 27, 2011	May 20, 2010	Apr. 24, 2009	Dec. 31, 2007
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares registered										4,000,000
Shares reclassified as authorized and unissued		1,700,000
4.0% Convertible Senior Notes [Member]
Class of Stock [Line Items]
Debt instrument, interest rate, stated percentage		4.00%							4.00%
Previous Stock Repurchase Program [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized						$ 1,000,000,000	$ 300,000,000
Stock repurchased, shares			1,300,000
Stock repurchased, value				$ 0
Weighted average price per share			$ 114.96
Stock repurchase program, remaining authorization			$ 150,000,000
New Stock Repurchase Program [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased, shares		5,300,000	2,700,000
Stock repurchased, value		$ 545,700,000	$ 300,000,000
Weighted average price per share		$ 102.14	$ 112.04
Stock repurchase program, remaining authorization		$ 154,400,000
New Stock Repurchase Program [Member] | Class A Common Stock [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized					$ 1,000,000,000
Stock repurchased, shares	42,163
Stock repurchased, value	$ 4,400,000
Weighted average price per share	$ 104.81
Stock repurchase program, remaining authorization	$ 1,000,000				$ 150,000,000
2010 Plan [Member] | Class A Common Stock [Member]
Class of Stock [Line Items]
Maximum issuance of shares		15,000,000						15,000,000