Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 751,653
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	439,297	$ 454,880
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	127,417	125,343
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	88,602	67,541
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	87,615	77,856
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8,722	5,081
Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	751,653	730,701
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Total	809,667	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	439,297	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	127,417	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	88,602	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	87,615	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	8,722	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	751,653	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	58,014	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	47,099
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	6,469
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	2,215
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	1,005
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	1,226
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred and common stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		53,513
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred and common stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		6,818
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		1,881
Fair Value, Measurements, Recurring Basis | Nonredeemable preferred and common stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		1,488
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	24,403	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	2,243	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	2,243	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	22,160	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	15,637
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	3,411
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	2,215
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 | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	897
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred and common stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		14,927
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred and common stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		3,190
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		0
Fair Value, Measurements, Recurring Basis | Level 1 | Nonredeemable preferred and common stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		0
Fair Value, Measurements, Recurring Basis | Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	765,825	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	424,998	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	127,417	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	81,219	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	87,615	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	8,722	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	729,971	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	35,854	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	31,462
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	3,058
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	1,005
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	329
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred and common stock | Financial services sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		38,586
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred and common stock | Utilities sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		3,628
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		1,881
Fair Value, Measurements, Recurring Basis | Level 2 | Nonredeemable preferred and common stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		1,488
Fair Value, Measurements, Recurring Basis | Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	19,439	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	12,056	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	7,383	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	19,439	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
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
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 | Other sectors
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 | Financial services 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 | Utilities 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
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
Fair Value, Measurements, Recurring Basis | Level 3 | Nonredeemable preferred and common stock | Other sectors
Fair value measurements on a recurring basis by asset class and level of input
Equity securities		$ 0