Income Taxes - Statutory Federal Income Tax Expense, Effective Income Tax Expense, Rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Amount
Statutory Federal Income Tax Benefit	$ 72.8	$ 77.9		$ 52.3
Tax-exempt Income and Dividends Received Deduction	4.8	5.3		4.6
Untaxed Earnings on Company-Owned Life Insurance	6.1	8.0		5.4
Tax credits	3.1	6.5		66.1
Stock-Based Compensation	(0.3)	(1.3)		0.3
Nondeductible Executive Compensation	(1.8)	(1.5)		(2.7)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount	(6.3)	0.0		0.0
Expense on Transactions	0.0	(11.5)		0.0
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount	27.4	0.0		0.0
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	(27.4)	0.0		0.0
Other, Net	(3.6)	1.0		(0.4)
Income Tax Benefit	$ 74.8	$ 84.4	[1]	$ 125.6	[1]
Rate
Statutory Federal Income Tax Benefit	21.00%	21.00%		21.00%
Tax-exempt Income and Dividends Received Deduction	1.40%	1.30%		1.90%
Effective Income Tax Rate Reconciliation, Untaxed Earnings on Company-Owned Life Insurance, Percent	1.80%	2.10%		2.20%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	0.90%	1.70%		27.00%
Stock-Based Compensation	0.10%	0.30%		(0.10%)
Nondeductible Executive Compensation	0.50%	0.40%		1.10%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent	(1.80%)	0.00%		0.00%
Expense on Transactions	0.00%	3.00%		0.00%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	7.90%	0.00%		0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	(7.90%)	0.00%		0.00%
Other, Net	1.10%	(0.30%)		0.20%
Effective Income Tax Benefit	21.60%	22.70%		50.90%

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.