Schedule II. Condensed Financial Information of Registrant (Condensed Statements of Income) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues
Net investment income	$ 72.7	$ 76.3	$ 88.1
Net realized and unrealized gains (losses) on investments	54.6	19.0	51.1
Total revenues	703.1	711.4	835.9
Expenses
Underwriting and general and administrative expenses	160.2	181.3	187.5
Interest and financing expenses	0.5	0.4	0.6
Total expenses	556.1	563.7	642.1
Income tax expense	27.7	27.9	36.7
Net income	$ 119.3	$ 119.8	$ 157.1
Earnings per common share for the stated periods (Note 17):
Basic	$ 4.22	$ 4.01	$ 4.89
Diluted	4.17	3.97	4.83
Cash dividends declared per common share and eligible equity plan holders	$ 1.00	$ 1.00	$ 0.88
Parent Company [Member]
Revenues
Net investment income	$ 0.6	$ 1.1	$ 3.7
Net realized and unrealized gains (losses) on investments	(0.2)	(1.9)	3.3
Total revenues	0.4	(0.8)	7.0
Expenses
Underwriting and general and administrative expenses	15.0	16.8	19.0
Interest and financing expenses	0.5	0.3	0.0
Total expenses	16.7	17.1	19.0
Loss before income taxes and equity in earnings of subsidiaries	(16.3)	(17.9)	(12.0)
Income tax expense	(3.1)	(3.2)	(2.5)
Net loss before equity in earnings of subsidiaries	(13.2)	(14.7)	(9.5)
Equity in earnings of subsidiaries	132.5	134.5	166.6
Net income	$ 119.3	$ 119.8	$ 157.1
Earnings per common share for the stated periods (Note 17):
Basic	$ 4.22	$ 4.01	$ 4.89
Diluted	$ 4.17	$ 3.97	$ 4.83