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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details) $ in Millions | Jun. 30, 2023 USD ($) security Security | Dec. 31, 2022 USD ($) | Dec. 31, 2022 security | Dec. 31, 2022 Security | Dec. 31, 2021 USD ($) security Security
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Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | $ 3,908.2 | $ 3,788.6 |  |  | $ 4,881.6
Gross unrealized gains | 2.4 | 3.6 |  |  | 95.3
Gross unrealized losses | (321.4) | (338.6) |  |  | (34.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (4.0) | (7.7) |  |  | (2.7)
Fixed income securities, available for sale amortized cost | 4,231.2 | 4,131.3 |  |  | 4,823.5
0-12 months, Fair Market Value | 1,175.6 | 2,321.4 |  |  | 2,014.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 32.5 | 145.4 |  |  | 30.9
Over 12 months, Fair Market Value | 2,582.7 | 1,323.7 |  |  | 72.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 3,758.3 | 3,645.1 |  |  | 2,086.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 321.4 | 339.0 |  |  | $ 34.5
Number of Securities | security | 1,193 |  | 1,166 |  | 644
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (288.9) | (193.6) |  |  | $ (3.6)
Total short-term investments — Available for sale |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Gross unrealized gains | 0.0 | 0.0 |  |  | 0.0
Gross unrealized losses | 0.0 | (0.4) |  |  | 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0 |  |  | 0.0
Total fixed income securities — Available for sale |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 4,077.5 | 3,840.6 |  |  | 4,891.7
Gross unrealized gains | 2.4 | 3.6 |  |  | 95.3
Gross unrealized losses | (321.4) | (339.0) |  |  | (34.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (4.0) | (7.7) |  |  | (2.7)
Fixed income securities, available for sale amortized cost | 4,400.5 | 4,183.7 |  |  | 4,833.6
Fixed income securities — Available for sale |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
0-12 months, Fair Market Value | 1,163.8 | 2,269.5 |  |  | 2,010.2
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 32.5 | 145.0 |  |  | 30.9
Over 12 months, Fair Market Value | 2,582.7 | 1,323.7 |  |  | 72.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 3,746.5 | 3,593.2 |  |  | 2,082.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 321.4 | 338.6 |  |  | $ 34.5
Number of Securities | Security | 1,188 |  |  | 1,165 | 637
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (288.9) | (193.6) |  |  | $ (3.6)
Short-term investments |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
0-12 months, Fair Market Value | 11.8 | 51.9 |  |  | 4.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 0.4 |  |  | 0.0
Over 12 months, Fair Market Value | 0.0 | 0.0 |  |  | 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 11.8 | 51.9 |  |  | 4.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 0.0 | 0.4 |  |  | $ 0.0
Number of Securities | 5 |  | 1 | 1 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 0.0 | 0.0 |  |  | $ 0.0
U.S. government |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 1,013.8 | 953.0 |  |  | 1,084.3
Gross unrealized gains | 0.0 | 0.0 |  |  | 24.0
Gross unrealized losses | (51.1) | (50.7) |  |  | (5.9)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0 |  |  | 0.0
Fixed income securities, available for sale amortized cost | 1,064.9 | 1,003.7 |  |  | 1,066.2
0-12 months, Fair Market Value | 494.8 | 741.1 |  |  | 309.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 12.8 | 30.6 |  |  | 4.0
Over 12 months, Fair Market Value | 519.1 | 203.4 |  |  | 29.3
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 1,013.9 | 944.5 |  |  | 338.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 51.1 | 50.7 |  |  | $ 5.9
Number of Securities | security | 105 |  | 118 |  | 51
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (38.3) | (20.1) |  |  | $ (1.9)
U.S. agency |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 8.2 | 8.8 |  |  | 21.5
Gross unrealized gains | 0.0 | 0.0 |  |  | 0.8
Gross unrealized losses | (0.4) | (0.4) |  |  | 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0 |  |  | 0.0
Fixed income securities, available for sale amortized cost | 8.6 | 9.2 |  |  | 20.7
0-12 months, Fair Market Value | 7.0 | 7.1 |  |  | 6.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.4 | 0.4 |  |  | 0.0
Over 12 months, Fair Market Value | 1.1 | 1.7 |  |  | 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 8.1 | 8.8 |  |  | 6.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 0.4 | 0.4 |  |  | $ 0.0
Number of Securities | security | 4 |  | 5 |  | 5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 0.0 | 0.0 |  |  | $ 0.0
Municipal |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 149.5 | 149.5 |  |  | 82.6
Gross unrealized gains | 0.0 | 0.0 |  |  | 2.0
Gross unrealized losses | (8.8) | (9.4) |  |  | (0.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (0.7) | (1.0) |  |  | (0.1)
Fixed income securities, available for sale amortized cost | 159.0 | 159.9 |  |  | 81.2
0-12 months, Fair Market Value | 90.9 | 133.1 |  |  | 29.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 3.7 | 7.1 |  |  | 0.5
Over 12 months, Fair Market Value | 58.7 | 16.4 |  |  | 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 149.6 | 149.5 |  |  | 29.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 8.8 | 9.4 |  |  | $ 0.5
Number of Securities | security | 60 |  | 59 |  | 20
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (5.1) | (2.3) |  |  | $ 0.0
Corporate |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 1,875.9 | 1,845.0 |  |  | 2,208.3
Gross unrealized gains | 2.4 | 3.5 |  |  | 47.5
Gross unrealized losses | (155.2) | (169.1) |  |  | (13.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (3.2) | (6.3) |  |  | (2.3)
Fixed income securities, available for sale amortized cost | 2,031.9 | 2,016.9 |  |  | 2,176.1
0-12 months, Fair Market Value | 467.4 | 1,104.7 |  |  | 804.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 13.1 | 77.6 |  |  | 11.7
Over 12 months, Fair Market Value | 1,254.7 | 568.2 |  |  | 23.6
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 1,722.1 | 1,672.9 |  |  | 828.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 155.2 | 169.1 |  |  | $ 13.0
Number of Securities | security | 727 |  | 701 |  | 400
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (142.1) | (91.5) |  |  | $ (1.3)
Non-U.S. government-backed corporate |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 110.9 | 110.4 |  |  | 136.8
Gross unrealized gains | 0.0 | 0.0 |  |  | 0.2
Gross unrealized losses | (8.7) | (8.8) |  |  | (1.9)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | (0.2) |  |  | 0.0
Fixed income securities, available for sale amortized cost | 119.6 | 119.4 |  |  | 138.5
0-12 months, Fair Market Value | 5.2 | 11.3 |  |  | 114.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 0.5 |  |  | 1.9
Over 12 months, Fair Market Value | 105.7 | 99.0 |  |  | 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 110.9 | 110.3 |  |  | 114.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 8.7 | 8.8 |  |  | $ 1.9
Number of Securities | security | 16 |  | 16 |  | 10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (8.7) | (8.3) |  |  | $ 0.0
Non-U.S government |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 269.2 | 213.6 |  |  | 250.7
Gross unrealized gains | 0.0 | 0.1 |  |  | 1.4
Gross unrealized losses | (12.0) | (11.5) |  |  | (1.2)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (0.1) | (0.2) |  |  | (0.3)
Fixed income securities, available for sale amortized cost | 281.3 | 225.2 |  |  | 250.8
0-12 months, Fair Market Value | 80.1 | 63.8 |  |  | 181.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 1.2 | 2.9 |  |  | 1.2
Over 12 months, Fair Market Value | 182.2 | 135.9 |  |  | 3.1
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 262.3 | 199.7 |  |  | 184.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 12.0 | 11.5 |  |  | $ 1.2
Number of Securities | security | 61 |  | 45 |  | 35
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (10.8) | (8.6) |  |  | $ 0.0
Asset-backed |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value |  |  |  |  | 0.5
Gross unrealized gains |  |  |  |  | 0.0
Gross unrealized losses |  |  |  |  | 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss |  |  |  |  | 0.0
Fixed income securities, available for sale amortized cost |  |  |  |  | 0.5
Non-agency commercial mortgage-backed securities |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 5.7 | 5.6 |  |  | 6.9
Gross unrealized gains | 0.0 | 0.0 |  |  | 0.3
Gross unrealized losses | (0.9) | (1.0) |  |  | 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0 |  |  | 0.0
Fixed income securities, available for sale amortized cost | 6.6 | 6.6 |  |  | 6.6
0-12 months, Fair Market Value | 0.0 | 5.6 |  |  |
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 0.0 | 1.0 |  |  |
Over 12 months, Fair Market Value | 5.6 | 0.0 |  |  |
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 5.6 | 5.6 |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 0.9 | 1.0 |  |  |
Number of Securities | security | 1 |  | 1 |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (0.9) | 0.0 |  |  |
Agency mortgage-backed |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 475.0 | 502.7 |  |  | 1,090.0
Gross unrealized gains | 0.0 | 0.0 |  |  | 19.1
Gross unrealized losses | (84.3) | (87.7) |  |  | (12.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0 |  |  | 0.0
Fixed income securities, available for sale amortized cost | 559.3 | 590.4 |  |  | 1,082.9
0-12 months, Fair Market Value | 18.4 | 202.8 |  |  | 564.5
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months | 1.3 | 24.9 |  |  | 11.6
Over 12 months, Fair Market Value | 455.6 | 299.1 |  |  | 16.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value | 474.0 | 501.9 |  |  | 580.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | $ 84.3 | 87.7 |  |  | $ 12.0
Number of Securities | security | 214 |  | 220 |  | 116
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ (83.0) | (62.8) |  |  | $ (0.4)
Total short-term investments — Available for sale |  |  |  |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |  |  |  |
Fair market value | 169.3 | 52.0 |  |  | 10.1
Fixed income securities, available for sale amortized cost | $ 169.3 | $ 52.4 |  |  | $ 10.1

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