Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 928,236	$ 730,701
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	566,425	454,880
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	112,179	125,343
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	120,201	67,541
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	110,447	77,856
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	18,984	5,081
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	1,022,326	797,834
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	566,425	454,880
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	112,179	125,343
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	120,201	67,541
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	110,447	77,856
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	18,984	5,081
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	928,236	730,701
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	94,090	67,133
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	76,575	53,513
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	8,742	6,818
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	3,068	508
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	2,699
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		1,881
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	800	980
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred and common stock | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		3,433
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	2,206
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	34,970	24,233
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	1,281	2,683
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 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	1,281	2,683
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	33,689	21,550
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	24,981	14,927
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	3,957	3,190
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	576	0
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	2,699
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		0
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	800	0
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred and common stock | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		3,433
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	676
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	961,132	761,956
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	559,319	443,873
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	111,242	125,343
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	100,739	64,220
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	110,447	77,856
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	18,984	5,081
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	900,731	716,373
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	60,401	45,583
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	51,594	38,586
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	4,785	3,628
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	2,492	508
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
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		1,881
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	980
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred and common stock | 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 | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,530
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	26,224	11,645
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	5,825	8,324
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	937	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	19,462	3,321
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	0	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	26,224	11,645
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	0	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	0	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
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred stock | Energy 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 | 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 | 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 | Nonredeemable preferred and common stock | Energy sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	$ 0