Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	$ 3,295,375	$ 3,299,515
Total Liabilities Measured at Fair Value	273,865	277,951
Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,961,541	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	289,612	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	658,129	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	362,870	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	96,134	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	476,838	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	168,609	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	1,104,451	951,380
Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	909,349	897,479
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	327,466	235,981
Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	145,077	52,439
Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	182,389	183,542
Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	6,368	6,480
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	273,865	277,951
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Assets Measured at Fair Value	118,820	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	80,379	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	80,379	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	38,441	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	38,441	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	3,176,555	3,062,345
Total Liabilities Measured at Fair Value	273,865	277,951
Level 2 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	2,881,162	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	209,233	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	658,129	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	362,870	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	96,134	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	476,838	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	168,609	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	1,104,451	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	909,349	897,479
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	289,025	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	106,636	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	182,389	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	6,368	0
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value	273,865	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