Schedule II Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023		Dec. 31, 2022
Cash flows from operating activities
Net income	$ 102.8	$ 45.0		$ 19.8	[1]
Changes in:
Other operating assets and liabilities	101.4	53.9		(109.4)	[2]
Net cash provided by operating activities	452.1	302.1		171.5	[2]
Cash flows from investing activities
Purchase of equity securities	(4.5)	(2.5)		(5.2)	[2]
Purchase of limited partnership interests	(74.8)	(207.2)		(356.4)	[2]
Other, net	7.2	9.2		(9.6)	[2]
Acquisition of business, net of cash acquired	0.0	0.0		(164.4)	[2]
Net cash used in investing activities	(135.8)	(107.4)		(214.6)	[2]
Cash flows from financing activities
Dividends paid to shareholders	(55.5)	(53.9)		(52.6)	[2]
Proceeds from issuance of 2023 Senior Notes due 2028	0.0	297.7		0.0	[2]
Principal repayment on Revolving Credit Facility	0.0	(249.0)		0.0	[2]
Proceeds from exercise of stock options	5.1	0.0		0.0	[2]
Withholding tax payments on RSUs tendered	(1.8)	(1.8)		(2.4)	[2]
Net cash used in financing activities	(307.9)	(207.8)		(47.8)	[2]
Net increase (decrease) in cash	8.4	(13.1)		(90.9)	[2]
Cash at beginning of year	29.7	42.8	[2]	133.7	[2]
Cash at end of year	38.1	29.7		42.8	[2]
Parent company
Cash flows from operating activities
Net income	102.8	45.0		19.8
Equity in net income of subsidiaries	(135.8)	(72.7)		(41.3)
Dividends received from subsidiaries	120.2	132.1		184.3
Changes in:
Income taxes	15.5	(5.8)		3.9
Other operating assets and liabilities	(8.1)	6.0		0.8
Other	2.0	1.2		(1.5)
Net cash provided by operating activities	96.6	105.8		166.0
Cash flows from investing activities
Purchase of equity securities	(1.2)	0.0		0.0
Purchase of limited partnership interests	(28.9)	0.0		0.0
Other, net	(12.0)	(1.3)		(0.7)
Capital contributions to subsidiaries	(2.0)	(98.0)		(35.0)
Acquisition of business, net of cash acquired	0.0	0.0		(172.3)
Net cash used in investing activities	(44.1)	(99.3)		(208.0)
Cash flows from financing activities
Dividends paid to shareholders	(55.6)	(53.9)		(52.6)
Proceeds from issuance of 2023 Senior Notes due 2028	0.0	296.5		0.0
Principal repayment on Revolving Credit Facility	0.0	(249.0)		0.0
Acquisition of treasury stock	(8.6)	(6.5)		(24.0)
Proceeds from exercise of stock options	5.1	0.0		0.0
Withholding tax payments on RSUs tendered	(1.8)	(1.8)		(2.4)
Proceeds for Share-based compensation	8.4	8.0		7.1
Net cash used in financing activities	(52.5)	(6.7)		(71.9)
Net increase (decrease) in cash	0.0	(0.2)		(113.9)
Cash at beginning of year	0.1	0.3		114.2
Cash at end of year	$ 0.1	$ 0.1		$ 0.3

[1] Recast for the adoption of ASU 2018-12.

[2] Recast for the adoption of ASU 2018-12.