Fair Value Measurements - Summary of Assets Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 1,095,523	$ 1,017,478
Trading securities:
Fixed maturity securities	3,812	5,063
Short-term investments	42,331	50,844
Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,016,818	941,077
State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	134,978	105,841
Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	139,057	150,798
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	421,406	378,448
Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	177,447	168,047
Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	62,375	65,014
U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	79,480	71,120
Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	2,075	1,809
Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	78,705	76,401
Recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,095,523	1,017,478
Trading securities:
Fixed maturity securities	3,812	5,063
Short-term investments	42,331	50,844
Recurring basis | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,016,818	941,077
Recurring basis | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	134,978	105,841
Recurring basis | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	139,057	150,798
Recurring basis | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	421,406	378,448
Recurring basis | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	177,447	168,047
Recurring basis | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	62,375	65,014
Recurring basis | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	79,480	71,120
Recurring basis | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	2,075	1,809
Recurring basis | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	78,705	76,401
Recurring basis | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	66,833	64,827
Recurring basis | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	11,872	11,574
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	90,002	81,305
Trading securities:
Fixed maturity securities	0	1,250
Short-term investments	1,000	1,100
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	78,864	70,465
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	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
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	78,864	70,465
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	11,138	10,840
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	11,138	10,840
Recurring basis | Significant Other Observable Inputs Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,000,521	931,173
Trading securities:
Fixed maturity securities	3,812	3,813
Short-term investments	41,331	49,744
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	932,954	865,612
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	134,978	105,841
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	139,057	150,798
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	421,406	378,448
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	172,447	163,047
Recurring basis | Significant Other Observable Inputs Level 2 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	62,375	65,014
Recurring basis | Significant Other Observable Inputs Level 2 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	616	655
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	2,075	1,809
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	67,567	65,561
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	66,833	64,827
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	734	734
Recurring basis | Significant Unobservable Inputs Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	5,000	5,000
Trading securities:
Fixed maturity securities	0	0
Short-term investments	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	5,000	5,000
Recurring basis | Significant Unobservable Inputs Level 3 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	5,000	5,000
Recurring basis | Significant Unobservable Inputs Level 3 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 0	$ 0