Basis of Presentation and Significant Accounting Policies - Narrative (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) plan segment $ / shares shares	Dec. 31, 2022 USD ($)		Dec. 31, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of reportable segments (in segments) | segment	4
Amortization of acquisition costs for property and casualty contracts, first term	6 months
Amortization of acquisition costs for property and casualty contracts, second term	12 months
Amortization of acquisition costs for supplemental and group benefits	6 years
Purchase price	$ 156.9
Amortization in 2024	14.5
Amortization in 2025	14.3
Amortization in 2026	14.2
Amortization in 2027	14.1
Amortization in 2028	14.1
Goodwill impairment	$ 0.0	$ 2.0		$ 0.0
Purchase of FHLB activity-based common stock as percentage of borrowing, required	4.50%
Purchase of FHLB activity-based common stock as percentage of borrowing, percentage, authorized	15.00%	25.00%
Repayment of FHLB funding agreements	$ 189.5	$ 149.0	[1]	362.0	[1]
FHLB advances	$ 904.5	792.5
Reverse repurchase agreement, cash to value percentage	95.00%
Securities sold under agreements to repurchase	$ 0.0	70.2
Share-based compensation expense	$ 7.5	6.9		6.6
Weighted average fair value of nonvested options outstanding (in usd per share) | $ / shares	$ 8.11
Unrecognized share-based compensation expense, nonvested options	$ 2.6
Antidilutive securities exclude from EPS computation (in shares) | shares	1,243,169
Minimum exercise price (in usd per share) | $ / shares	$ 32.13
Maximum exercise price (in usd per share) | $ / shares	$ 42.95
Qualified Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of retirement plans | plan	2
Nonqualified Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of retirement plans | plan	2
401(k) plan assets | Qualified Plan | Pension Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Employer matching contribution as percent of employees' gross pay	3.00%
Percentage of employer match	5.00%
Vesting period for Company contributions	5 years
Fair Value
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Securities sold under agreements to repurchase	$ 0.0	73.9
Employee stock option
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Share-based compensation expense	1.4	$ 1.2		$ 1.2
Restricted stock units
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Unrecognized share-based compensation expense, equity instruments other than options	8.9
HMLIC and NTA
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Proceeds from FHLB funding agreements	301.5
Repayment of FHLB funding agreements	$ 189.5
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Purchase of FHLB activity-based common stock as percentage of borrowing, required	2.00%
Real estate property | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Property, plant and equipment, useful life	20 years
Real estate property | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Property, plant and equipment, useful life	45 years
Lease hold improvements and other property and equipment | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Property, plant and equipment, useful life	3 years
Lease hold improvements and other property and equipment | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Property, plant and equipment, useful life	10 years
Value of business acquired
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Purchase price	$ 64.8
Amortization in 2024	5.4
Amortization in 2025	5.1
Amortization in 2026	4.7
Amortization in 2027	4.4
Amortization in 2028	4.1
Value of distribution acquired | NTA
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Purchase price	36.0
Amortization in 2024	2.9
Amortization in 2025	2.9
Amortization in 2026	2.9
Amortization in 2027	2.9
Amortization in 2028	2.9
Value of agency relationships | NTA
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amortization in 2024	1.4
Amortization in 2025	1.2
Amortization in 2026	1.0
Amortization in 2027	0.9
Amortization in 2028	0.8
Value of agency relationship	7.2
Value of customer relationships | Benefit Consultants Group Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Purchase price	48.4
Amortization in 2024	4.8
Amortization in 2025	5.1
Amortization in 2026	5.5
Amortization in 2027	5.9
Amortization in 2028	$ 6.3

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements