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		Jun. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 357,961	$ 191,386
Amortized Cost/Cost		370,421	194,522
Gross Unrealized Losses and OTTI	[1]	(12,460)	(3,136)
Less than or equal to six months | Fixed maturity AFS securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		205,185	168,922
Amortized Cost/Cost		210,714	170,752
Gross Unrealized Losses and OTTI	[1]	(5,529)	(1,830)
Less than or equal to six months | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		137,459	109,579
Amortized Cost/Cost		142,333	110,819
Gross Unrealized Losses and OTTI	[1]	(4,874)	(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		24,941	20,762
Amortized Cost/Cost		25,219	20,791
Gross Unrealized Losses and OTTI	[1]	(278)	(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		19,885	28,994
Amortized Cost/Cost		20,177	29,409
Gross Unrealized Losses and OTTI	[1]	(292)	(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		10,002	2,054
Amortized Cost/Cost		10,008	2,098
Gross Unrealized Losses and OTTI	[1]	(6)	(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		222	7,533
Amortized Cost/Cost		225	7,635
Gross Unrealized Losses and OTTI	[1]	(3)	(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		12,676
Amortized Cost/Cost		12,752
Gross Unrealized Losses and OTTI	[1]	(76)
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		131,177	9,483
Amortized Cost/Cost		136,195	9,630
Gross Unrealized Losses and OTTI	[1]	(5,018)	(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		92,512
Amortized Cost/Cost		96,184
Gross Unrealized Losses and OTTI	[1]	(3,672)
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		3,920
Amortized Cost/Cost		4,000
Gross Unrealized Losses and OTTI	[1]	(80)
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		27,357	26
Amortized Cost/Cost		28,386	26
Gross Unrealized Losses and OTTI	[1]	(1,029)
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		7,388
Amortized Cost/Cost		7,625
Gross Unrealized Losses and OTTI	[1]	(237)
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		5,588	5,690
Amortized Cost/Cost		5,791	5,791
Gross Unrealized Losses and OTTI	[1]	(203)	(101)
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			3,767
Amortized Cost/Cost			3,813
Gross Unrealized Losses and OTTI	[1]		(46)
Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		21,599	12,981
Amortized Cost/Cost		23,512	14,140
Gross Unrealized Losses and OTTI	[1]	(1,913)	(1,159)
Greater than one year | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		8,173	8,498
Amortized Cost/Cost		9,441	9,440
Gross Unrealized Losses and OTTI	[1]	(1,268)	(942)
Greater than one year | Fixed maturity AFS securities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		2,296	2,610
Amortized Cost/Cost		2,518	2,768
Gross Unrealized Losses and OTTI	[1]	(222)	(158)
Greater than one year | Fixed maturity AFS securities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		996	986
Amortized Cost/Cost		1,018	1,020
Gross Unrealized Losses and OTTI	[1]	(22)	(34)
Greater than one year | Fixed maturity AFS securities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		24	6
Amortized Cost/Cost		25	6
Gross Unrealized Losses and OTTI	[1]	(1)
Greater than one year | Fixed maturity AFS securities | Municipals
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		887	881
Amortized Cost/Cost		904	906
Gross Unrealized Losses and OTTI	[1]	(17)	$ (25)
Greater than one year | Fixed maturity AFS securities | Government and government agencies - Foreign
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		3,635
Amortized Cost/Cost		3,815
Gross Unrealized Losses and OTTI	[1]	$ (180)

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