Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Total available for sale investments		$ 2,967,909
Gross Unrealized Gains		18,767
Gross Unrealized Losses		(48,855)
Amortized Cost		2,997,997
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		2,961,541	3,057,054
Gross Unrealized Gains		18,767	44,158
Gross Unrealized Losses		(48,855)	(14,512)
Amortized Cost		2,991,629	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		289,612	393,563
Gross Unrealized Gains		955	2,081
Gross Unrealized Losses		(4,189)	(2,014)
Amortized Cost		292,846	393,496
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total available for sale investments		658,129	814,632
Gross Unrealized Gains		10,410	20,136
Gross Unrealized Losses		(4,870)	(1,423)
Amortized Cost		652,589	795,919
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		362,870	407,619
Gross Unrealized Gains		1,091	2,352
Gross Unrealized Losses		(14,497)	(5,414)
Amortized Cost		376,276	410,681
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		96,134	54,104
Gross Unrealized Gains		464	606
Gross Unrealized Losses		(431)	(79)
Amortized Cost		96,101	53,577
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		476,838	328,753
Gross Unrealized Gains		737	2,138
Gross Unrealized Losses		(2,749)	(663)
Amortized Cost		478,850	327,278
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		168,609	160,904
Gross Unrealized Gains		807	2,354
Gross Unrealized Losses		(2,817)	(1,182)
Amortized Cost		170,619	159,732
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		1,104,451	951,380
Gross Unrealized Gains		3,099	7,450
Gross Unrealized Losses		(20,494)	(7,338)
Amortized Cost		1,121,846	951,268
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total available for sale investments	[1]	909,349	897,479
Gross Unrealized Gains	[1]	4,303	14,491
Gross Unrealized Losses	[1]	(19,302)	(3,737)
Amortized Cost	[1]	924,348	886,725
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total available for sale investments		6,368	6,480
Gross Unrealized Gains			3
Amortized Cost		$ 6,368	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.