Accounting Developments New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Other, Net			$ (8.5)	$ (6.9)
Proceeds from Sales and Maturities of Other Investments			257.4	99.5
Compensation Expense	$ 220.2	$ 217.8	441.9	426.8
Other Expenses	220.8	204.3	445.0	433.1
Other Long-term Investments	687.5		687.5			$ 646.8
Deferred Income Tax	85.5		85.5			199.0
Accumulated Other Comprehensive Income (Loss)	(276.2)		(276.2)			127.5
Retained Earnings	$ 10,012.4		$ 10,012.4			9,542.2
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Other, Net				6.9
Proceeds from Sales and Maturities of Other Investments				99.5
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		217.8		426.8
Other Expenses		204.3		433.1
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				7.3
Proceeds from Sales and Maturities of Other Investments				(7.3)
Effect of Change | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		1.9		3.9
Other Expenses		(1.9)		(3.9)
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				(0.4)
Proceeds from Sales and Maturities of Other Investments				106.8
Historical Accounting Method | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		215.9		422.9
Other Expenses		$ 206.2		$ 437.0