Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	$ 3,368,417	$ 3,299,515
Total Liabilities Measured at Fair Value	274,348	277,951
Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,989,473	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	245,285	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	631,514	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	344,016	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	124,191	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	540,566	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	186,251	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	1,195,024	951,380
Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	917,650	897,479
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	371,680	235,981
Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	190,722	52,439
Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	180,958	183,542
Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	7,264	6,480
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	274,348	277,951
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	85,509	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	45,093	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	45,093	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	40,416	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	40,416	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	3,282,908	3,062,345
Total Liabilities Measured at Fair Value	274,348	277,951
Level 2 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,944,380	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	200,192	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	631,514	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	344,016	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	124,191	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	540,566	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	186,251	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	1,195,024	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	917,650	897,479
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	331,264	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	150,306	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	180,958	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	7,264	0
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	274,348	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