Summary of Significant Accounting Policies Statement Presentation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net premiums earned	$ 6,345.8	$ 5,919.9	[1]	$ 5,651.1	[1]
Title, escrow, and other fees	391.9	321.1		289.8
Underwriting Acquisition And Other Expenses	$ 3,942.4	3,525.4		3,317.7
Revision of Prior Period, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net premiums earned		421.5		397.7
Title, escrow, and other fees		174.7		160.6
Underwriting Acquisition And Other Expenses		$ 246.8		$ 237.0

[1] Certain reclassifications were made to net premiums earned, title, escrow, and other fees, and underwriting, acquisition, and other expenses in 2019 and 2018 to conform to the presentation adopted in 2020.