Fair Value Measurements - Summary of Assets Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	$ 1,332,042	$ 1,184,202
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	1,332,042	1,184,202
Equity securities:
Total equity securities	87,528	78,385
Short-term investments	24,463	81,966
Fixed maturity securities
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	1,332,042	1,184,202
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	1,332,042	1,184,202
State and municipal
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	138,250	149,295
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	138,250	149,295
Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	249,758	204,109
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	249,758	204,109
Corporate
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	606,274	524,768
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	606,274	524,768
Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	220,076	197,025
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	220,076	197,025
U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	115,633	107,193
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	115,633	107,193
Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	2,051	1,812
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	2,051	1,812
Recurring basis
Equity securities:
Short-term investments	24,463	81,966
Recurring basis | Fixed maturity securities
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	1,332,042	1,184,202
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	1,332,042	1,184,202
Recurring basis | State and municipal
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	138,250	149,295
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	138,250	149,295
Recurring basis | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	249,758	204,109
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	249,758	204,109
Recurring basis | Corporate
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	606,274	524,768
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	606,274	524,768
Recurring basis | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	220,076	197,025
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	220,076	197,025
Recurring basis | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	115,633	107,193
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	115,633	107,193
Recurring basis | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	2,051	1,812
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	2,051	1,812
Recurring basis | Equity securities
Equity securities:
Total equity securities	87,528	78,385
Recurring basis | Preferred stock
Equity securities:
Total equity securities	67,255	60,740
Recurring basis | Common stock
Equity securities:
Total equity securities	20,273	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:
Total fixed maturity securities, available-for-sale	115,094	106,651
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	115,094	106,651
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | State and municipal
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Corporate
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	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:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	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:
Total fixed maturity securities, available-for-sale	115,094	106,651
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	115,094	106,651
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Equity securities
Equity securities:
Total equity securities	15,413	16,674
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Preferred stock
Equity securities:
Total equity securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Common stock
Equity securities:
Total equity securities	15,413	16,674
Recurring basis | Significant Other Observable Inputs Level 2
Equity securities:
Short-term investments	24,463	80,716
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	1,216,948	1,073,323
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	1,216,948	1,073,323
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	138,250	149,295
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	138,250	149,295
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	249,758	204,109
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	249,758	204,109
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	606,274	524,768
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	606,274	524,768
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	220,076	192,797
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	220,076	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:
Total fixed maturity securities, available-for-sale	539	542
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	539	542
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	2,051	1,812
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	2,051	1,812
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Equity securities:
Total equity securities	69,016	61,497
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Equity securities:
Total equity securities	67,255	60,740
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Equity securities:
Total equity securities	1,761	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:
Total fixed maturity securities, available-for-sale	0	4,228
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	4,228
Recurring basis | Significant Unobservable Inputs Level 3 | State and municipal
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Corporate
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	4,228
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	4,228
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:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Total fixed maturity securities, available-for-sale	0	0
Fixed maturity securities, available-for-sale, at fair value (amortized cost: 2019 – $1,302,946; 2018 – $1,199,409)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Equity securities
Equity securities:
Total equity securities	3,099	214
Recurring basis | Significant Unobservable Inputs Level 3 | Preferred stock
Equity securities:
Total equity securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Common stock
Equity securities:
Total equity securities	$ 3,099	$ 214