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Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Financial Assets |  |  |  |
Beginning balance | $ 478,800,000 | $ 420,000,000.0 | $ 425,300,000 | $ 371,400,000
Transfers into Level 3 | 0 | 33,500,000 | 36,900,000 | 179,300,000
Transfers out of Level 3 | (3,700,000) | (34,800,000) | (3,700,000) | (42,900,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | 200,000 | (900,000) | 200,000 | (3,200,000)
Net unrealized gains/losses included in OCI | (4,900,000) | (16,600,000) | (6,700,000) | (41,600,000)
Purchases | 19,200,000 | 16,500,000 | 54,600,000 | 21,400,000
Issuances | 0 | 0 | 0 | 0
Sales | 0 | 0 | (7,700,000) | (2,100,000)
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | (12,000,000.0) | (15,400,000) | (21,300,000) | (80,000,000.0)
Ending balance | 477,600,000 | 402,300,000 | 477,600,000 | 402,300,000
Financial Liabilities |  |  |  |
Beginning balance | 83,600,000 | 97,800,000 | 91,300,000 | 111,400,000
Transfers into Level 3 | 0 | 0 | 0 | 0
Transfers out of Level 3 | 0 | 0 | 0 | 0
Total gains or losses |  |  |  |
Net gains/losses included in net income | (3,200,000) | (6,300,000) | (5,600,000) | (13,900,000)
Net unrealized gains/losses included in OCI | 400,000 | 0 | (300,000) | (2,400,000)
Purchases | 0 | 0 | 0 | 0
Issuances | 2,000,000.0 | 2,700,000 | 6,300,000 | 4,800,000
Sales | 0 | 0 | 0 | 0
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | (4,500,000) | (1,400,000) | (13,400,000) | (7,100,000)
Ending balance | 78,300,000 | 92,800,000 | 78,300,000 | 92,800,000
Level 3 |  |  |  |
Total gains or losses |  |  |  |
Net gains/losses included in net income | 200,000 | (900,000) | 200,000 | (3,200,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | (3,200,000) | (6,300,000) | (5,600,000) | (13,900,000)
Municipal bonds |  |  |  |
Financial Assets |  |  |  |
Beginning balance | 63,900,000 | 51,000,000.0 | 54,400,000 | 60,800,000
Transfers into Level 3 | 0 | 0 | 0 | 0
Transfers out of Level 3 | 0 | 0 | 0 | (3,200,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | 0 | 0 | 0 | 0
Net unrealized gains/losses included in OCI | 5,900,000 | (5,300,000) | 6,500,000 | (11,600,000)
Purchases | 0 | 200,000 | 9,500,000 | 200,000
Issuances | 0 | 0 | 0 | 0
Sales | 0 | 0 | 0 | 0
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | (100,000) | 7,400,000 | (700,000) | 7,100,000
Ending balance | 69,700,000 | 53,300,000 | 69,700,000 | 53,300,000
Corporate bonds |  |  |  |
Financial Assets |  |  |  |
Beginning balance | 311,500,000 | 270,500,000 | 261,300,000 | 210,300,000
Transfers into Level 3 | 0 | 20,500,000 | 36,100,000 | 144,000,000.0
Transfers out of Level 3 | (3,700,000) | (34,800,000) | (3,700,000) | (34,900,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | 0 | 0 | 0 | 0
Net unrealized gains/losses included in OCI | (12,800,000) | (7,400,000) | (14,100,000) | (18,700,000)
Purchases | 17,500,000 | 8,400,000 | 43,200,000 | 13,300,000
Issuances | 0 | 0 | 0 | 0
Sales | 0 | 0 | (7,700,000) | 0
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | (7,300,000) | (17,400,000) | (9,900,000) | (74,200,000)
Ending balance | 305,200,000 | 239,800,000 | 305,200,000 | 239,800,000
Mortgage Backed and Other Asset- Backed Securities |  |  |  |
Financial Assets |  |  |  |
Beginning balance | 101,400,000 | 97,100,000 | 107,600,000 | 98,900,000
Transfers into Level 3 | 0 | 12,200,000 | 800,000 | 34,500,000
Transfers out of Level 3 | 0 | 0 | 0 | (4,800,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | 0 | (800,000) | 0 | (3,100,000)
Net unrealized gains/losses included in OCI | 2,000,000.0 | (3,900,000) | 900,000 | (11,300,000)
Purchases | 1,700,000 | 7,900,000 | 1,900,000 | 7,900,000
Issuances | 0 | 0 |  | 0
Sales | 0 | 0 |  | (2,100,000)
Settlements | 0 | 0 |  | 0
Paydowns, maturities and distributions | (4,600,000) | (5,300,000) | (10,700,000) | (12,800,000)
Ending balance | 100,500,000 | 107,200,000 | 100,500,000 | 107,200,000
Total Fixed Maturity Securities |  |  |  |
Financial Assets |  |  |  |
Beginning balance | 476,800,000 | 418,600,000 | 423,300,000 | 370,000,000.0
Transfers into Level 3 | 0 | 32,700,000 | 36,900,000 | 178,500,000
Transfers out of Level 3 | (3,700,000) | (34,800,000) | (3,700,000) | (42,900,000)
Total gains or losses |  |  |  |
Net gains/losses included in net income | 0 | (800,000) | 0 | (3,100,000)
Net unrealized gains/losses included in OCI | (4,900,000) | (16,600,000) | (6,700,000) | (41,600,000)
Purchases | 19,200,000 | 16,500,000 | 54,600,000 | 21,400,000
Issuances | 0 | 0 | 0 | 0
Sales | 0 | 0 | (7,700,000) | (2,100,000)
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | (12,000,000.0) | (15,300,000) | (21,300,000) | (79,900,000)
Ending balance | 475,400,000 | 400,300,000 | 475,400,000 | 400,300,000
Equity securities |  |  |  |
Financial Assets |  |  |  |
Beginning balance | 2,000,000.0 | 1,400,000 | 2,000,000.0 | 1,400,000
Transfers into Level 3 | 0 | 800,000 | 0 | 800,000
Transfers out of Level 3 | 0 | 0 | 0 | 0
Total gains or losses |  |  |  |
Net gains/losses included in net income | 200,000 | (100,000) | 200,000 | (100,000)
Net unrealized gains/losses included in OCI | 0 | 0 | 0 | 0
Purchases | 0 | 0 | 0 | 0
Issuances | 0 | 0 | 0 | 0
Sales | 0 | 0 | 0 | 0
Settlements | 0 | 0 | 0 | 0
Paydowns, maturities and distributions | 0 | (100,000) | 0 | (100,000)
Ending balance | $ 2,200,000 | $ 2,000,000.0 | $ 2,200,000 | $ 2,000,000.0

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