Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 946,085
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	573,165	$ 566,425
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	115,462	110,447
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	89,324	120,201
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	139,922	112,179
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	23,920	18,984
U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4,292
Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	946,085	928,236
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	1,033,828	1,022,326
Fair Value, Measurements, Recurring Basis | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	573,165	566,425
Fair Value, Measurements, Recurring Basis | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	115,462	110,447
Fair Value, Measurements, Recurring Basis | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	89,324	120,201
Fair Value, Measurements, Recurring Basis | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	139,922	112,179
Fair Value, Measurements, Recurring Basis | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	23,920	18,984
Fair Value, Measurements, Recurring Basis | U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4,292
Fair Value, Measurements, Recurring Basis | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	946,085	928,236
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	87,743	94,090
Fair Value, Measurements, Recurring Basis | Equity securities | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	71,722	76,575
Fair Value, Measurements, Recurring Basis | Equity securities | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	6,259	8,742
Fair Value, Measurements, Recurring Basis | Equity securities | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	6,448	2,206
Fair Value, Measurements, Recurring Basis | Equity securities | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	3,314	3,068
Fair Value, Measurements, Recurring Basis | Equity securities | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		2,699
Fair Value, Measurements, Recurring Basis | Equity securities | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		800
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	1,634	34,970
Fair Value, Measurements, Recurring Basis | Level 1 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	1,281
Fair Value, Measurements, Recurring Basis | Level 1 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
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
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
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
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 1 | U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	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
Available-for-sale securities	0	1,281
Fair Value, Measurements, Recurring Basis | Level 1 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,634	33,689
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
Equity securities	1,624	24,981
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
Equity securities	0	3,957
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
Equity securities	10	676
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
Equity securities	0	576
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
Equity securities		2,699
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
Equity securities		800
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	1,000,313	961,132
Fair Value, Measurements, Recurring Basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	567,909	559,319
Fair Value, Measurements, Recurring Basis | Level 2 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	115,462	110,447
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
Available-for-sale securities	73,596	100,739
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
Available-for-sale securities	131,108	111,242
Fair Value, Measurements, Recurring Basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	23,920	18,984
Fair Value, Measurements, Recurring Basis | Level 2 | U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	4,292
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
Available-for-sale securities	916,287	900,731
Fair Value, Measurements, Recurring Basis | Level 2 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	84,026	60,401
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
Equity securities	68,015	51,594
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
Equity securities	6,259	4,785
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
Equity securities	6,438	1,530
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
Equity securities	3,314	2,492
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
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
Equity securities		0
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	31,881	26,224
Fair Value, Measurements, Recurring Basis | Level 3 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	5,256	5,825
Fair Value, Measurements, Recurring Basis | Level 3 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
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
Available-for-sale securities	15,728	19,462
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
Available-for-sale securities	8,814	937
Fair Value, Measurements, Recurring Basis | Level 3 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3 | U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
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
Available-for-sale securities	29,798	26,224
Fair Value, Measurements, Recurring Basis | Level 3 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	2,083	0
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
Equity securities	2,083	0
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
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
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
Equity securities	$ 0	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
Equity securities		0
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
Equity securities		$ 0