Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 804,825
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	469,655	$ 454,880
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	119,599	125,343
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	106,542	67,541
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	96,288	77,856
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	12,741	5,081
Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	804,825	730,701
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	880,214	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	469,655	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	119,599	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	106,542	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	96,288	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	12,741	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	804,825	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	75,389	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	60,149	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	7,712	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	2,599	508
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	1,035	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		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	3,894	3,433
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	32,092	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	2,104	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	2,104	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	29,988	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	20,943	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	4,259	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	892	0
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	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		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	3,894	3,433
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	824,086	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	462,635	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	119,599	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	88,476	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	96,029	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	12,741	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	779,480	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	44,606	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	39,206	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	3,453	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	912	508
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	1,035	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		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	0
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	24,036	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	4,916	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	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	18,066	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	259	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	23,241	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	795	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	795	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	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
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	$ 0