Accounting Developments New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Other Assets	$ 906.3	$ 789.0
Deferred Income Tax	30.2	109.9
Other Liabilities	1,790.3	1,762.8
Income Tax Payable	268.3	24.0
Retained Earnings	$ 10,083.1	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
Restatement Adjustment | Accounting Standards Update 2016-02
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)
Restatement Adjustment | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Deferred Tax Liabilities, Net			$ (0.8)
Retained Earnings			$ 14.5