Schedule II. Condensed Financial Information of Registrant (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net income	$ 119.3	$ 119.8	$ 157.1
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Realized gains on investments	(54.6)	(19.0)	(51.1)
Stock-based compensation	9.1	9.7	10.1
Amortization of premium on investments, net	8.5	9.8	8.7
Deferred income tax expense	6.7	(0.4)	8.6
Change in operating assets and liabilities:
Accounts payable, accrued expenses and other liabilities	(2.5)	(4.0)	(8.1)
Federal income taxes	(7.9)	22.0	(7.4)
Other	6.3	(1.0)	(11.8)
Net cash provided by (used in) operating activities	10.8	33.0	122.5
Investing activities
Purchases of fixed maturity securities	(516.6)	(645.6)	(359.0)
Purchase of equity securities	(199.5)	(179.5)	(240.8)
Proceeds from sale of equity securities	135.9	243.4	232.4
Purchases of short-term investments	(12.5)	(135.9)	0.0
Proceeds from sale of fixed maturity securities	206.7	349.5	163.0
Proceeds from maturities and redemptions of fixed maturity securities	373.5	359.2	309.9
Net change in unsettled investment purchases and sales	3.4	(3.6)	(24.7)
Net cash (used in) provided by investing activities	(1.7)	84.3	49.2
Financing activities
Acquisition of common stock	(42.6)	(99.4)	(67.5)
Cash transactions related to stock-based compensation	(2.7)	(1.8)	(2.5)
Dividends paid to stockholders and eligible equity plan holders	(29.0)	(30.5)	(28.3)
Net cash used in financing activities	(94.4)	(111.9)	(118.5)
Cash and cash equivalents at the beginning of the period	160.4
Cash and cash equivalents at the end of the period	75.1	160.4
Parent Company [Member]
Operating activities
Net income	119.3	119.8	157.1
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in net income of subsidiaries	40.9	39.9	70.4
Realized gains on investments	0.2	1.9	(3.3)
Stock-based compensation	9.1	9.7	10.1
Amortization of premium on investments, net	0.1	0.1	0.0
Deferred income tax expense	(0.2)	0.8	(2.8)
Change in operating assets and liabilities:
Accounts payable, accrued expenses and other liabilities	0.0	(2.2)	2.3
Federal income taxes	(0.7)	3.5	19.2
Other assets	0.4	0.5	0.7
Intercompany payable/receivable	(0.1)	3.5	(3.2)
Other	0.1	0.3	0.0
Net cash provided by (used in) operating activities	86.3	96.4	108.3
Investing activities
Purchases of fixed maturity securities	0.0	(3.2)	(9.3)
Purchase of equity securities	(35.0)	(3.0)	(42.0)
Proceeds from sale of equity securities	10.3	29.0	56.0
Purchases of short-term investments	0.0	0.0	0.0
Proceeds from sale of fixed maturity securities	0.4	14.9	4.3
Proceeds from maturities and redemptions of fixed maturity securities	0.0	3.8	3.8
Proceeds from maturities of short-term investments	0.0	0.0	25.0
Net change in unsettled investment purchases and sales	5.8	(4.4)	(5.0)
Capital Contributions to subsidiary	0.0	(0.4)	(73.6)
Net cash (used in) provided by investing activities	(18.5)	36.7	(40.8)
Financing activities
Acquisition of common stock	(42.6)	(99.4)	(67.5)
Cash transactions related to stock-based compensation	(2.7)	(1.8)	(2.5)
Dividends paid to stockholders and eligible equity plan holders	(28.7)	(30.8)	(28.9)
Net cash used in financing activities	(74.0)	(132.0)	(98.9)
Net increase (decrease) in cash and cash equivalents	(6.2)	1.1	(31.4)
Cash and cash equivalents at the beginning of the period	11.0	9.9	41.3
Cash and cash equivalents at the end of the period	$ 4.8	$ 11.0	$ 9.9