Accounting Developments New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Other, Net			$ (1.2)	$ (8.2)
Proceeds from Sales and Maturities of Other Investments			350.6	161.7
Compensation Expense	$ 224.2	$ 223.8	666.1	650.6
Other Expenses	212.9	191.5	657.9	624.6
Other Long-term Investments	694.2		694.2			$ 646.8
Deferred Income Tax	0.0		0.0			199.0
Accumulated Other Comprehensive Income (Loss)	(868.5)		(868.5)			127.5
Retained Earnings	$ 9,670.8		$ 9,670.8			9,542.2
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Other, Net				8.2
Proceeds from Sales and Maturities of Other Investments				161.7
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		223.8		650.6
Other Expenses		191.5		624.6
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Other Long-term Investments					$ 643.0	646.8
Deferred Income Tax					198.2	199.0
Accumulated Other Comprehensive Income (Loss)					110.0	127.5
Retained Earnings					9,556.7	$ 9,542.2
Effect of Change | Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Other, Net				10.6
Proceeds from Sales and Maturities of Other Investments				(10.6)
Effect of Change | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		2.0		5.9
Other Expenses		(2.0)		(5.9)
Effect of Change | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Other Long-term Investments					(3.8)
Deferred Income Tax					(0.8)
Accumulated Other Comprehensive Income (Loss)					(17.5)
Retained Earnings					$ 14.5
Historical Accounting Method | Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Other, Net				(2.4)
Proceeds from Sales and Maturities of Other Investments				172.3
Historical Accounting Method | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		221.8		644.7
Other Expenses		$ 193.5		$ 630.5