Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	$ 1,433,626	$ 1,184,202
Equity securities:
Equity securities	80,735	78,385
Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,433,626	1,184,202
State and municipal
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	167,101	149,295
Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	264,146	204,109
Corporate
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	632,221	524,768
Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	252,457	197,025
U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	115,667	107,193
Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	2,034	1,812
Recurring basis
Equity securities:
Short-term investments	156,925	81,966
Recurring basis | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,433,626	1,184,202
Recurring basis | State and municipal
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	167,101	149,295
Recurring basis | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	264,146	204,109
Recurring basis | Corporate
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	632,221	524,768
Recurring basis | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	252,457	197,025
Recurring basis | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	115,667	107,193
Recurring basis | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	2,034	1,812
Recurring basis | Equity securities
Equity securities:
Equity securities	80,735	78,385
Recurring basis | Preferred stock
Equity securities:
Equity securities	62,747	60,740
Recurring basis | Common stock
Equity securities:
Equity securities	17,988	17,645
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1
Equity securities:
Short-term investments	0	1,250
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	115,173	106,651
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | State and municipal
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Corporate
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	115,173	106,651
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Equity securities
Equity securities:
Equity securities	14,669	16,674
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Preferred stock
Equity securities:
Equity securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Common stock
Equity securities:
Equity securities	14,669	16,674
Recurring basis | Significant Other Observable Inputs Level 2
Equity securities:
Short-term investments	156,925	80,716
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,318,453	1,073,323
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	167,101	149,295
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	264,146	204,109
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	632,221	524,768
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	252,457	192,797
Recurring basis | Significant Other Observable Inputs Level 2 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	494	542
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	2,034	1,812
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Equity securities:
Equity securities	66,023	61,497
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Equity securities:
Equity securities	62,747	60,740
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Equity securities:
Equity securities	3,276	757
Recurring basis | Significant Unobservable Inputs Level 3
Equity securities:
Short-term investments	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	4,228
Recurring basis | Significant Unobservable Inputs Level 3 | State and municipal
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Corporate
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		4,228	$ 4,700
Recurring basis | Significant Unobservable Inputs Level 3 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Equity securities
Equity securities:
Equity securities	43	214
Recurring basis | Significant Unobservable Inputs Level 3 | Preferred stock
Equity securities:
Equity securities	$ 0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Common stock
Equity securities:
Equity securities		$ 214