Fair Value - Fair Value Measurements on a Recurring Basis by Asset Class and Level of Input (Details) $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) fund
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 730,701
Total	797,834
U.S. Treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		$ 208,412
States and political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		159,023
Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	454,880	249,947
Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	125,343	4,609
Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	67,541	46,515
Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	77,856	64,239
Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	5,081	15,778
Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	730,701	748,523
Level 1
Fair value measurements on a recurring basis by asset class and level of input
Total	24,233
Level 1 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,683
Level 2
Fair value measurements on a recurring basis by asset class and level of input
Total	761,956
Level 2 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	716,373
Level 3
Fair value measurements on a recurring basis by asset class and level of input
Total	11,645
Level 3 | Available-for-sale securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,645
Fair Value, Measurements, Recurring Basis
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	730,701
Total	797,834	763,582
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		208,412
Fair Value, Measurements, Recurring Basis | States and political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		159,023
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	454,880	249,947
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	125,343	4,609
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	67,541	46,515
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	77,856	64,239
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	5,081	15,778
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		748,523
Fair Value, Measurements, Recurring Basis | Equity securities
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	67,133	11,853
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	53,513	$ 11,853
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	6,818
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 | 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 | 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 | Real estate
Fair value measurements on a recurring basis by asset class and level of input
Number of real estate funds reported under the fair value option | fund		1
Fair Value, Measurements, Recurring Basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,683
Total	24,233	$ 1,809
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 | States and political subdivisions
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 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,683	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 | 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		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	21,550	1,809
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	14,927	1,809
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	3,190
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	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 2
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	716,373
Total	761,956	745,990
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		208,412
Fair Value, Measurements, Recurring Basis | Level 2 | States and political subdivisions
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		159,023
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	443,873	237,370
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	125,343	4,609
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	64,220	46,515
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	77,856	64,239
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	5,081	15,778
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		735,946
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	45,583	10,044
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	38,586	10,044
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	3,628
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 | 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 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 3
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,645
Total	11,645	12,577
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 | States and political subdivisions
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 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	8,324	12,577
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	3,321	0
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		12,577
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
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 | Communications sector
Fair value measurements on a recurring basis by asset class and level of input
Equity securities	$ 0
Fair Value, Measurements, Recurring Basis | Fair Value Measured at Net Asset Value Per Share | Real estate
Fair value measurements on a recurring basis by asset class and level of input
Other limited partnership investments		$ 3,206