Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 936,798	$ 928,236
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	564,689	566,425
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	115,442	110,447
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	112,292	120,201
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	93,652	112,179
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	29,332	18,984
U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	21,391
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	1,030,803	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	936,798	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	564,689	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	115,442	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	112,292	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	93,652	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	29,332	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	21,391
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	94,005	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	75,212	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	10,470	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	2,295	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	2,161	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	915	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	2,952	2,206
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	30,285	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	2,461	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	2,461	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 | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	27,824	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	20,628	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	3,594	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	607	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	1,123	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	915	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	957	676
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	969,972	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	904,791	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	556,150	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	114,692	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	95,159	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	90,611	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	26,788	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	21,391
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	65,181	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	53,584	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	6,876	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	1,688	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	1,038	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	1,995	1,530
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	30,546	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	29,546	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	6,078	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	750	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	17,133	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	3,041	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	2,544	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	1,000	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	1,000	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