Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets:
Short-term investments	$ 1,032.6	$ 860.8
Other assets	981.5	874.8
Total assets	26,501.4	25,159.4
Liabilities:
Debt	1,591.2	1,597.0
Total liabilities	20,090.7	18,986.2
Common Shareholders' Equity:
Common stock	278.3	296.9
Additional paid-in capital	678.7	1,141.8
Retained earnings	5,644.3	5,321.8
Accumulated other comprehensive loss	(132.4)	(517.8)
Unallocated ESSOP Shares	58.2	69.5
Total Common Shareholders' Equity	6,410.7	6,173.2
Liabilities and Common Shareholders' Equity:
Total liabilities, preferred stock and common shareholders' equity	26,501.4	25,159.4
Parent Company [Member]
Assets:
Bond and notes	10.5	10.5
Short-term investments	21.1	35.1
Cash	5.0	2.0
Investments in, and indebtedness of related parties	8,027.7	7,823.0
Other assets	116.6	104.8
Total assets	8,181.0	7,975.5
Liabilities:
Accounts payable and accrued expenses	69.6	98.8
Debt	1,591.2	1,589.9
Accounts Payable, Related Party, Type [Extensible Enumeration]	109.4	120.5
Total liabilities	1,770.3	1,809.2
Liabilities and Common Shareholders' Equity:
Total liabilities, preferred stock and common shareholders' equity	$ 8,181.0	$ 7,975.5