Summary of Significant Accounting Policies Change in Accounting Principle (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 4,051.2		$ 4,386.0
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, Allowance for Credit Loss	20.4	$ 15.5
Reinsurance Recoverable, Allowance for Credit Loss	15.0	14.5
Total credit loss allowance relating to financial assets	$ 35.4	30.1
Revision of Prior Period, Accounting Standards Update, Adjustment | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 2.3