Accounting Developments New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable	$ 95.2	$ 103.7
Proceeds from Sales and Maturities of Other Investments	82.3	46.5
Compensation Expense	221.7	209.0
Other Expenses	224.2	228.8
Other Long-term Investments	706.5			$ 646.8
Deferred Income Tax	128.9			199.0
Accumulated Other Comprehensive Income (Loss)	(82.4)			127.5
Retained Earnings	$ 9,777.8			9,542.2
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		4.4
Proceeds from Sales and Maturities of Other Investments		46.5
Accounting Standards Update 2016-15 | Scenario, Previously Reported
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		2.2
Proceeds from Sales and Maturities of Other Investments		48.7
Accounting Standards Update 2016-15 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		2.2
Proceeds from Sales and Maturities of Other Investments		(2.2)
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		209.0
Other Expenses		228.8
Accounting Standards Update 2017-07 | Scenario, Previously Reported
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		207.0
Other Expenses		230.8
Accounting Standards Update 2017-07 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		2.0
Other Expenses		$ (2.0)
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
Accounting Standards Update 2016-01 | Restatement Adjustment
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