New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle
Other Assets	$ 908.0	$ 789.0
Deferred Income Tax	0.0	109.9
Other Liabilities	1,827.2	1,762.8
Income Tax Payable	250.2	24.0
Retained Earnings	$ 10,308.9	9,863.1
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Other Assets		906.7
Deferred Income Tax		109.5
Other Liabilities		1,884.8
Income Tax Payable		22.7
Retained Earnings		9,859.7
Accounting Standards Update 2016-02 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle
Other Assets		117.7
Deferred Income Tax		(0.4)
Other Liabilities		122.0
Income Tax Payable		(1.3)
Retained Earnings		$ (3.4)