v3.26.3
Investments - Schedule of Composition of the Fair Values of Investments by Credit Rating (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2026
Dec. 31, 2025
Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 2,792.7 $ 2,640.4
Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Short-term investments    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 268.7 $ 111.3
Short-term investments | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 100.00% 100.00%
AAA | Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 286.1 $ 216.1
AAA | Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 10.00% 9.00%
AAA | Short-term investments    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 0.4 $ 0.4
AAA | Short-term investments | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 0.00% 0.00%
AA | Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 886.1 $ 891.2
AA | Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 32.00% 34.00%
AA | Short-term investments    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 266.9 $ 109.9
AA | Short-term investments | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 100.00% 99.00%
A | Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 1,182.2 $ 1,118.8
A | Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 42.00% 42.00%
A | Short-term investments    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 0.8 $ 0.7
A | Short-term investments | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 0.00% 1.00%
BBB | Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 413.7 $ 382.8
BBB | Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 15.00% 14.00%
BBB | Short-term investments    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 0.6 $ 0.3
BBB | Short-term investments | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 0.00% 0.00%
Below BBB | Fixed maturity securities    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Fair value $ 24.6 $ 31.5
Below BBB | Fixed maturity securities | Credit Rating | Credit Concentration Risk    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Concentration risk, percentage 1.00% 1.00%