Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Total available for sale investments		$ 2,977,638	$ 3,299,515
Gross Unrealized Gains		23,581	57,655
Gross Unrealized Losses		(40,142)	(16,184)
Amortized Cost		2,994,199	3,258,044
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		2,970,934	3,057,054
Gross Unrealized Gains		23,581	44,158
Gross Unrealized Losses		(40,139)	(14,512)
Amortized Cost		2,987,492	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		357,303	393,563
Gross Unrealized Gains		1,192	2,081
Gross Unrealized Losses		(4,082)	(2,014)
Amortized Cost		360,193	393,496
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total available for sale investments		721,232	814,632
Gross Unrealized Gains		11,912	20,136
Gross Unrealized Losses		(4,554)	(1,423)
Amortized Cost		713,874	795,919
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		379,354	407,619
Gross Unrealized Gains		1,389	2,352
Gross Unrealized Losses		(13,466)	(5,414)
Amortized Cost		391,431	410,681
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		63,430	54,104
Gross Unrealized Gains		499	606
Gross Unrealized Losses		(266)	(79)
Amortized Cost		63,197	53,577
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		369,337	328,753
Gross Unrealized Gains		1,361	2,138
Gross Unrealized Losses		(1,701)	(663)
Amortized Cost		369,677	327,278
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		161,391	160,904
Gross Unrealized Gains		1,088	2,354
Gross Unrealized Losses		(2,051)	(1,182)
Amortized Cost		162,354	159,732
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		973,512	951,380
Gross Unrealized Gains		4,337	7,450
Gross Unrealized Losses		(17,484)	(7,338)
Amortized Cost		986,659	951,268
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total available for sale investments	[1]	918,887	897,479
Gross Unrealized Gains	[1]	6,140	14,491
Gross Unrealized Losses	[1]	(14,019)	(3,737)
Amortized Cost	[1]	926,766	886,725
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total available for sale investments		6,704	6,480
Gross Unrealized Gains			3
Gross Unrealized Losses		(3)
Amortized Cost		$ 6,707	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.