Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		$ 3,420,163		$ 3,128,563
Total Liabilities Measured at Fair Value		277,951		280,316
Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		3,057,054	[1]	2,635,882
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		393,563	[1]	273,776
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		814,632	[1]	547,415
Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		407,619	[1]	487,364
Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		54,104	[1]	20,530
Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		328,753	[1]	314,601
Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		160,904	[1]	154,139
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		951,380	[1]	976,634
Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	[2]	897,479	[1]	838,057
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		235,981	[1]	349,142
Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		52,439	[1]	164,087
Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542	[1]	185,055
Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		127,128	[1]	143,539
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		277,951		280,316
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		357,818		355,330
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		178,251		47,704
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		178,251		47,704
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 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		52,439		164,087
Level 1 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		52,439		164,087
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		127,128		143,539
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]
Fixed Maturities and Equity Securities, at Fair Value		3,062,345		2,773,233
Total Liabilities Measured at Fair Value		277,951		280,316
Level 2 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		2,878,803		2,588,178
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		215,312		226,072
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		814,632		547,415
Level 2 | Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		407,619		487,364
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		54,104		20,530
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		328,753		314,601
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		160,904		154,139
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		951,380		976,634
Level 2 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		897,479		838,057
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542		185,055
Level 2 | 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 2 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542		185,055
Level 2 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		277,951		280,316
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, 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 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

[1] Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.

[2] Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.