Shareholders' Equity (Narrative) (Details) - Class A Common Stock [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 01, 2018	Feb. 16, 2018	Jan. 12, 2017	Jun. 04, 2015	Apr. 27, 2011	May 20, 2010	Dec. 31, 2007
Class of Stock [Line Items]
Shares registered										4,000,000
Shares reclassified as authorized and unissued	1,200,000
Stock Repurchase Program One [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized								$ 300.0
Stock repurchased, shares			1,300,000
Weighted average price per share			$ 114.96
Stock repurchase program, remaining authorization			$ 150.0
Stock Repurchase Program Two [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized							$ 1,000.0
Stock repurchased, shares	42,163	5,300,000	2,700,000
Stock repurchased, value	$ 4.4	$ 545.7	$ 300.0
Weighted average price per share	$ 104.81	$ 102.14	$ 112.04
Stock repurchase program, remaining authorization		$ 154.4				$ 150.0
Stock Repurchase Program Three [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized						$ 1,000.0
Stock repurchased, shares	5,800,000
Stock repurchased, value	$ 850.0
Weighted average price per share	$ 146.17
Stock Repurchase Program Three [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization					$ 150.0
Stock Repurchase Program Four [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, authorized					$ 1.0
Stock repurchase program, remaining authorization				$ 1,000.0
2010 Plan [Member]
Class of Stock [Line Items]
Maximum issuance of shares	15,000,000								15,000,000
Common stock, shares issued	487,963	0	0
2015 Plan [Member]
Class of Stock [Line Items]
Common stock, shares issued	0	0	0