Summary of Significant Accounting Policies Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Deferred beginning of year	$ 325.4	$ 316.3	$ 297.8
Acquisition costs deferred; additions	504.2	541.4	508.1
Amortization charged to income	(501.5)	(532.2)	(489.6)
Change for the year	2.6	9.1	18.5
Deferred end of year	328.0	325.4	316.3
Commissions, net of reinsurance [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	326.0	360.8	332.2
Premium taxes [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	127.4	130.2	123.5
Salaries and other marketing expenses [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	$ 50.7	$ 50.3	$ 52.3