Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	$ 3,232,032	$ 3,299,515
Total Liabilities Measured at Fair Value	276,923	277,951
Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,970,934	3,057,054
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	357,303	393,563
Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	721,232	814,632
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	379,354	407,619
Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	63,430	54,104
Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	369,337	328,753
Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	161,391	160,904
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	973,512	951,380
Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	918,887	897,479
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	254,394	235,981
Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	59,734	52,439
Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	194,660	183,542
Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	6,704	6,480
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	276,923	277,951
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	168,035	237,170
Total Liabilities Measured at Fair Value	0	0
Level 1 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	130,902	178,251
Level 1 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	130,902	178,251
Level 1 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Agency Residential Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	37,133	52,439
Level 1 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	37,133	52,439
Level 1 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 1 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	6,480
Level 1 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	3,063,997	3,062,345
Total Liabilities Measured at Fair Value	276,923	277,951
Level 2 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,840,032	2,878,803
Level 2 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	226,401	215,312
Level 2 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	721,232	814,632
Level 2 | Fixed Maturities | Agency Residential Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	379,354	407,619
Level 2 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	63,430	54,104
Level 2 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	369,337	328,753
Level 2 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	161,391	160,904
Level 2 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	973,512	951,380
Level 2 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	918,887	897,479
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	217,261	183,542
Level 2 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	22,601	0
Level 2 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	194,660	183,542
Level 2 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	6,704	0
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	276,923	277,951
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	0	0
Total Liabilities Measured at Fair Value	0	0
Level 3 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Agency Residential Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	0	0
Level 3 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	$ 0	$ 0