Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 918,322	$ 928,236
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	549,815	566,425
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	106,649	112,179
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	113,965	120,201
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	115,207	110,447
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	22,291	18,984
U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,395
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	1,011,253	1,022,326
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	918,322	928,236
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	549,815	566,425
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	106,649	112,179
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	113,965	120,201
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	115,207	110,447
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	22,291	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	10,395
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	92,931	94,090
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	73,520	76,575
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	9,265	8,742
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	3,620	3,068
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	3,124	2,699
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	894	800
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	2,508	2,206
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	34,928	34,970
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	3,165	1,281
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	3,165	1,281
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 | 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 | 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 | 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 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	31,763	33,689
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	22,274	24,981
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	3,498	3,957
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,110	576
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	3,124	2,699
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Industrial sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	894	800
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	863	676
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	952,540	961,132
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	892,462	900,731
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	541,190	559,319
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	106,176	111,242
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	97,724	100,739
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	115,207	110,447
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	21,770	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	10,395
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	60,078	60,401
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	50,156	51,594
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	5,767	4,785
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Consumer sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	2,510	2,492
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Communications 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 2 | Nonredeemable preferred stock | Industrial 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 2 | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,645	1,530
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	23,785	26,224
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	22,695	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	5,460	5,825
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	473	937
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	16,241	19,462
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 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	521	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 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,090	0
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,090	0
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | 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 | Nonredeemable preferred stock | 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 | Nonredeemable preferred stock | Communications 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 | Nonredeemable preferred stock | Industrial 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 | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	$ 0	$ 0