Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	$ 915,237
Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	915,237	$ 894,661
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	566,967	553,382
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	110,488	102,537
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	72,032	66,054
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	150,394	150,415
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	15,356	22,273
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	994,753	967,221
Fair Value, Measurements, Recurring Basis | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	915,237	894,661
Fair Value, Measurements, Recurring Basis | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	566,967	553,382
Fair Value, Measurements, Recurring Basis | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	110,488	102,537
Fair Value, Measurements, Recurring Basis | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	72,032	66,054
Fair Value, Measurements, Recurring Basis | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	150,394	150,415
Fair Value, Measurements, Recurring Basis | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	15,356	22,273
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	79,516	72,560
Fair Value, Measurements, Recurring Basis | Equity securities | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	65,071	61,084
Fair Value, Measurements, Recurring Basis | Equity securities | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	7,291	5,708
Fair Value, Measurements, Recurring Basis | Equity securities | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	4,131	3,576
Fair Value, Measurements, Recurring Basis | Equity securities | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	2,288	1,854
Fair Value, Measurements, Recurring Basis | Equity securities | Technology sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	500
Fair Value, Measurements, Recurring Basis | Equity securities | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	177
Fair Value, Measurements, Recurring Basis | Equity securities | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	58	338
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	854	0
Fair Value, Measurements, Recurring Basis | Level 1 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	854	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	796	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Technology sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	58	0
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	978,426	944,662
Fair Value, Measurements, Recurring Basis | Level 2 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	903,720	875,881
Fair Value, Measurements, Recurring Basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	562,862	549,696
Fair Value, Measurements, Recurring Basis | Level 2 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	110,488	102,537
Fair Value, Measurements, Recurring Basis | Level 2 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	64,625	55,144
Fair Value, Measurements, Recurring Basis | Level 2 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	150,389	146,231
Fair Value, Measurements, Recurring Basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	15,356	22,273
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	74,706	68,781
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	60,819	57,305
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	7,291	5,708
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	4,131	3,576
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	2,288	1,854
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Technology sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	177
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	338
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	15,473	22,559
Fair Value, Measurements, Recurring Basis | Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	11,517	18,780
Fair Value, Measurements, Recurring Basis | Level 3 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	4,105	3,686
Fair Value, Measurements, Recurring Basis | Level 3 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	7,407	10,910
Fair Value, Measurements, Recurring Basis | Level 3 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	5	4,184
Fair Value, Measurements, Recurring Basis | Level 3 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Total available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	3,956	3,779
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	3,456	3,779
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Technology sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	500
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	0
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Total equity securities	$ 0	$ 0