Available-for-Sale Investments and Equity Securities Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Total available for sale investments		$ 3,093,156
Gross Unrealized Gains		18,059
Gross Unrealized Losses		(60,550)
Amortized Cost		3,135,647
Total available for sale investments			$ 3,299,515
Gross Unrealized Gains			57,655
Gross Unrealized Losses			(16,184)
Amortized Cost			3,258,044
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		3,082,923	3,057,054
Gross Unrealized Gains		18,059	44,158
Gross Unrealized Losses		(60,549)	(14,512)
Amortized Cost		3,125,413	3,027,408
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Total available for sale investments		239,776	393,563
Gross Unrealized Gains		901	2,081
Gross Unrealized Losses		(3,957)	(2,014)
Amortized Cost		242,832	393,496
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total available for sale investments		646,551	814,632
Gross Unrealized Gains		10,734	20,136
Gross Unrealized Losses		(3,448)	(1,423)
Amortized Cost		639,265	795,919
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		335,542	407,619
Gross Unrealized Gains		843	2,352
Gross Unrealized Losses		(13,118)	(5,414)
Amortized Cost		347,817	410,681
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		138,373	54,104
Gross Unrealized Gains		604	606
Gross Unrealized Losses		(530)	(79)
Amortized Cost		138,299	53,577
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		531,991	328,753
Gross Unrealized Gains		1,018	2,138
Gross Unrealized Losses		(8,040)	(663)
Amortized Cost		539,013	327,278
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		188,201	160,904
Gross Unrealized Gains		858	2,354
Gross Unrealized Losses		(2,816)	(1,182)
Amortized Cost		190,159	159,732
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		1,194,107	951,380
Gross Unrealized Gains		3,323	7,450
Gross Unrealized Losses		(24,504)	(7,338)
Amortized Cost		1,215,288	951,268
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total available for sale investments	[1]	1,002,489	897,479
Gross Unrealized Gains	[1]	3,101	14,491
Gross Unrealized Losses	[1]	(28,640)	(3,737)
Amortized Cost	[1]	1,028,028	886,725
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total available for sale investments		10,233	6,480
Gross Unrealized Gains		0	3
Gross Unrealized Losses		(1)	0
Amortized Cost		$ 10,234	6,477
Equity Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments			235,981
Gross Unrealized Gains			13,494
Gross Unrealized Losses			(1,672)
Amortized Cost			224,159
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Total available for sale investments			52,439
Gross Unrealized Gains			7,423
Gross Unrealized Losses			(112)
Amortized Cost			45,128
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Total available for sale investments			183,542
Gross Unrealized Gains			6,071
Gross Unrealized Losses			(1,560)
Amortized Cost			$ 179,031

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