Summary of Significant Accounting Policies Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Deferred beginning of year	$ 316.3	$ 297.8	$ 274.0
Acquisition costs deferred; additions	541.4	508.1	477.8
Amortization charged to income	(532.2)	(489.6)	(454.0)
Change for the year	9.1	18.5	23.7
Deferred end of year	325.4	316.3	297.8
Commissions, net of reinsurance [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	360.8	332.2	308.7
Premium taxes [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	130.2	123.5	117.1
Salaries and other marketing expenses [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	$ 50.3	$ 52.3	$ 51.9