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		Mar. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 306,718	$ 191,386
Amortized Cost/Cost		316,570	194,522
Gross Unrealized Losses and OTTI	[1]	(9,852)	(3,136)
Less than or equal to six months | Fixed maturity AFS securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		276,910	168,922
Amortized Cost/Cost		284,274	170,752
Gross Unrealized Losses and OTTI	[1]	(7,364)	(1,830)
Less than or equal to six months | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		189,755	109,579
Amortized Cost/Cost		194,764	110,819
Gross Unrealized Losses and OTTI	[1]	(5,009)	(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		28,429	20,762
Amortized Cost/Cost		28,683	20,791
Gross Unrealized Losses and OTTI	[1]	(254)	(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		41,667	28,994
Amortized Cost/Cost		42,654	29,409
Gross Unrealized Losses and OTTI	[1]	(987)	(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		(1,938)	2,054
Amortized Cost/Cost		(1,127)	2,098
Gross Unrealized Losses and OTTI	[1]	(811)	(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		16,468	7,533
Amortized Cost/Cost		16,768	7,635
Gross Unrealized Losses and OTTI	[1]	(300)	(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		2,529
Amortized Cost/Cost		2,532
Gross Unrealized Losses and OTTI	[1]	(3)
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		13,824	9,483
Amortized Cost/Cost		14,562	9,630
Gross Unrealized Losses and OTTI	[1]	(738)	(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		3,397
Amortized Cost/Cost		3,681
Gross Unrealized Losses and OTTI	[1]	(284)
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		2,798	26
Amortized Cost/Cost		2,969	26
Gross Unrealized Losses and OTTI	[1]	(171)
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		2,029
Amortized Cost/Cost		2,121
Gross Unrealized Losses and OTTI	[1]	(92)
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		5,600	5,690
Amortized Cost/Cost		5,791	5,791
Gross Unrealized Losses and OTTI	[1]	(191)	(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		15,984	12,981
Amortized Cost/Cost		17,734	14,140
Gross Unrealized Losses and OTTI	[1]	(1,750)	(1,159)
Greater than one year | Fixed maturity AFS securities | Corporate securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		8,132	8,498
Amortized Cost/Cost		9,441	9,440
Gross Unrealized Losses and OTTI	[1]	(1,309)	(942)
Greater than one year | Fixed maturity AFS securities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		2,344	2,610
Amortized Cost/Cost		2,550	2,768
Gross Unrealized Losses and OTTI	[1]	(206)	(158)
Greater than one year | Fixed maturity AFS securities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		969	986
Amortized Cost/Cost		1,019	1,020
Gross Unrealized Losses and OTTI	[1]	(50)	(34)
Greater than one year | Fixed maturity AFS securities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		5	6
Amortized Cost/Cost		5	6
Greater than one year | Fixed maturity AFS securities | Municipals
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		884	881
Amortized Cost/Cost		905	906
Gross Unrealized Losses and OTTI	[1]	(21)	$ (25)
Greater than one year | Fixed maturity AFS securities | Government and government agencies - Foreign
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		3,650
Amortized Cost/Cost		3,814
Gross Unrealized Losses and OTTI	[1]	$ (164)

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