Estimated Fair Value and Gross Unrealized Losses and Other-Than-Temporary Impairments related to Fixed Maturity and Equity Available-for-Sale Securities in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 371,107	$ 191,386
Amortized Cost/Cost		383,833	194,522
Gross Unrealized Losses and OTTI	[1]	(12,726)	(3,136)
Less than or equal to six months | Fixed maturity AFS securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		107,338	168,922
Amortized Cost/Cost		108,395	170,752
Gross Unrealized Losses and OTTI	[1]	(1,057)	(1,830)
Less than or equal to six months | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		77,544	109,579
Amortized Cost/Cost		78,325	110,819
Gross Unrealized Losses and OTTI	[1]	(781)	(1,240)
Less than or equal to six months | Fixed maturity AFS securities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		13,352	20,762
Amortized Cost/Cost		13,514	20,791
Gross Unrealized Losses and OTTI	[1]	(162)	(29)
Less than or equal to six months | Fixed maturity AFS securities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		3,940	28,994
Amortized Cost/Cost		3,967	29,409
Gross Unrealized Losses and OTTI	[1]	(27)	(415)
Less than or equal to six months | Fixed maturity AFS securities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		10,139	2,054
Amortized Cost/Cost		10,197	2,098
Gross Unrealized Losses and OTTI	[1]	(58)	(44)
Less than or equal to six months | Fixed maturity AFS securities | Government and government agencies - United States
Gain (Loss) on Investments [Line Items]
Estimated Fair Value			7,533
Amortized Cost/Cost			7,635
Gross Unrealized Losses and OTTI	[1]		(102)
Less than or equal to six months | Fixed maturity AFS securities | Government and government agencies - Foreign
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		2,363
Amortized Cost/Cost		2,392
Gross Unrealized Losses and OTTI	[1]	(29)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		237,272	9,483
Amortized Cost/Cost		246,811	9,630
Gross Unrealized Losses and OTTI	[1]	(9,539)	(147)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		166,680
Amortized Cost/Cost		174,211
Gross Unrealized Losses and OTTI	[1]	(7,531)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		19,172
Amortized Cost/Cost		19,543
Gross Unrealized Losses and OTTI	[1]	(371)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		41,334	26
Amortized Cost/Cost		42,637	26
Gross Unrealized Losses and OTTI	[1]	(1,303)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		40
Amortized Cost/Cost		41
Gross Unrealized Losses and OTTI	[1]	(1)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Redeemable preferred stock
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		5,375	5,690
Amortized Cost/Cost		5,791	5,791
Gross Unrealized Losses and OTTI	[1]	(416)	(101)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Government and government agencies - United States
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		7,559
Amortized Cost/Cost		7,846
Gross Unrealized Losses and OTTI	[1]	(287)
Greater than six months but less than or equal to one year | Fixed maturity AFS securities | Government and government agencies - Foreign
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		2,487	3,767
Amortized Cost/Cost		2,533	3,813
Gross Unrealized Losses and OTTI	[1]	(46)	(46)
Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		26,497	12,981
Amortized Cost/Cost		28,627	14,140
Gross Unrealized Losses and OTTI	[1]	(2,130)	(1,159)
Greater than one year | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		11,885	8,498
Amortized Cost/Cost		13,108	9,440
Gross Unrealized Losses and OTTI	[1]	(1,223)	(942)
Greater than one year | Fixed maturity AFS securities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		858	2,610
Amortized Cost/Cost		1,000	2,768
Gross Unrealized Losses and OTTI	[1]	(142)	(158)
Greater than one year | Fixed maturity AFS securities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		3,849	986
Amortized Cost/Cost		3,988	1,020
Gross Unrealized Losses and OTTI	[1]	(139)	(34)
Greater than one year | Fixed maturity AFS securities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		21	6
Amortized Cost/Cost		21	6
Greater than one year | Fixed maturity AFS securities | Government and government agencies - Foreign
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		3,609
Amortized Cost/Cost		3,816
Gross Unrealized Losses and OTTI	[1]	(207)
Greater than one year | Fixed maturity AFS securities | Municipals
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		900	881
Amortized Cost/Cost		903	906
Gross Unrealized Losses and OTTI	[1]	$ (3)	$ (25)

[1] Subsequent unrealized gains (losses) on OTTI securities are included in Net unrealized OTTI on securities in the Statements of Comprehensive Income (Loss).