Fair Value Measurements - Summary of Assets Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 1,105,396	$ 1,098,620
Trading securities:
Fixed maturity securities	0	3,808
Short-term investments	25,140	36,804
Recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,105,396	1,098,620
Trading securities:
Fixed maturity securities	0	3,808
Short-term investments	25,140	36,804
Recurring basis | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,021,387	1,016,098
Recurring basis | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	138,373	144,366
Recurring basis | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	184,751	158,661
Recurring basis | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	430,345	413,721
Recurring basis | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	174,442	182,611
Recurring basis | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	5,689	35,847
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	85,797	78,874
Recurring basis | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,990	2,018
Recurring basis | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	84,009	82,522
Recurring basis | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	64,866	66,281
Recurring basis | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	19,143	16,241
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	100,538	93,772
Trading securities:
Fixed maturity securities	0	0
Short-term investments	1,000	1,000
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	85,227	78,265
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	85,227	78,265
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	15,311	15,507
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	15,311	15,507
Recurring basis | Significant Other Observable Inputs Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,000,178	1,000,168
Trading securities:
Fixed maturity securities	0	3,808
Short-term investments	24,140	35,804
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	931,480	933,153
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	138,373	144,366
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	184,751	158,661
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	430,345	413,721
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	169,762	177,931
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	5,689	35,847
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	570	609
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	1,990	2,018
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	68,698	67,015
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	64,866	66,281
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	3,832	734
Recurring basis | Significant Unobservable Inputs Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	4,680	4,680
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	4,680	4,680
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	4,680	4,680
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