Schedule II Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Cash flows from operating activities
Net income	$ 162.1	$ 102.8	$ 45.0
Realized investment gains	(13.0)	(17.3)	(24.0)
Changes in:
Income taxes	(34.7)	(22.1)	(14.0)
Other operating assets and liabilities	219.1	101.4	53.9
Net cash provided by operating activities	553.2	452.1	302.1
Cash flows from investing activities
Purchase of fixed maturity securities	(1,347.8)	(1,161.5)	(596.7)
Purchase of equity securities	(2.3)	(4.5)	(2.5)
Purchase of limited partnership interests	(68.8)	(74.8)	(207.2)
Sale of limited partnership interests	98.8	119.5	41.4
Other, net	(9.8)	7.2	9.2
Net cash used in investing activities	(252.1)	(135.8)	(107.4)
Cash flows from financing activities
Dividends paid to shareholders	(57.1)	(55.5)	(53.9)
Proceeds from issuance of Senior Notes	295.2	0.0	297.7
Principal repayment on Revolving Credit Facility	0.0	0.0	(249.0)
Principal repayment on Senior Notes	(250.0)	0.0	0.0
Proceeds from exercise of stock options	5.3	5.1	0.0
Withholding tax payments on RSUs tendered	(3.4)	(1.8)	(1.8)
Net cash used in financing activities	(311.7)	(307.9)	(207.8)
Net increase (decrease) in cash	(10.6)	8.4	(13.1)
Cash at beginning of year	38.1	29.7	42.8
Cash at end of year	27.5	38.1	29.7
Parent company
Cash flows from operating activities
Net income	162.1	102.8	45.0
Realized investment gains	1.9	0.0	0.0
Equity in net income of subsidiaries	(198.5)	(135.8)	(72.7)
Dividends received from subsidiaries	116.0	120.2	132.1
Changes in:
Income taxes	1.2	15.5	(5.8)
Other operating assets and liabilities	17.7	(8.1)	6.0
Other	(2.4)	2.0	1.2
Net cash provided by operating activities	98.0	96.6	105.8
Cash flows from investing activities
Purchase of fixed maturity securities	(16.9)	0.0	0.0
Purchase of equity securities	(0.8)	(1.2)	0.0
Purchase of limited partnership interests	(29.4)	(28.9)	0.0
Sale of limited partnership interests	34.8	0.0	0.0
Other, net	(62.6)	(12.0)	(1.3)
Capital contributions to subsidiaries	(1.0)	(2.0)	(98.0)
Net cash used in investing activities	(75.9)	(44.1)	(99.3)
Cash flows from financing activities
Dividends paid to shareholders	(57.1)	(55.6)	(53.9)
Proceeds from issuance of Senior Notes	295.2	0.0	296.5
Principal repayment on Revolving Credit Facility	0.0	0.0	(249.0)
Principal repayment on Senior Notes	(250.0)	0.0	0.0
Acquisition of treasury stock	(20.8)	(8.6)	(6.5)
Proceeds from exercise of stock options	5.3	5.1	0.0
Withholding tax payments on RSUs tendered	(3.4)	(1.8)	(1.8)
Proceeds for Share-based compensation	9.1	8.4	8.0
Net cash used in financing activities	(21.7)	(52.5)	(6.7)
Net increase (decrease) in cash	0.4	0.0	(0.2)
Cash at beginning of year	0.1	0.1	0.3
Cash at end of year	$ 0.5	$ 0.1	$ 0.1