Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Total available for sale investments		$ 2,996,737
Gross Unrealized Gains		15,436
Gross Unrealized Losses		(53,802)
Amortized Cost		3,035,103
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		2,989,473	3,057,054
Gross Unrealized Gains		15,436	44,158
Gross Unrealized Losses		(53,798)	(14,512)
Amortized Cost		3,027,835	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		245,285	393,563
Gross Unrealized Gains		1,031	2,081
Gross Unrealized Losses		(3,879)	(2,014)
Amortized Cost		248,133	393,496
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total available for sale investments		631,514	814,632
Gross Unrealized Gains		7,569	20,136
Gross Unrealized Losses		(7,534)	(1,423)
Amortized Cost		631,479	795,919
Fixed Maturities | Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		344,016	407,619
Gross Unrealized Gains		874	2,352
Gross Unrealized Losses		(17,219)	(5,414)
Amortized Cost		360,361	410,681
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		124,191	54,104
Gross Unrealized Gains		521	606
Gross Unrealized Losses		(493)	(79)
Amortized Cost		124,163	53,577
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		540,566	328,753
Gross Unrealized Gains		554	2,138
Gross Unrealized Losses		(4,033)	(663)
Amortized Cost		544,045	327,278
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		186,251	160,904
Gross Unrealized Gains		696	2,354
Gross Unrealized Losses		(3,126)	(1,182)
Amortized Cost		188,681	159,732
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total available for sale investments		1,195,024	951,380
Gross Unrealized Gains		2,645	7,450
Gross Unrealized Losses		(24,871)	(7,338)
Amortized Cost		1,217,250	951,268
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total available for sale investments	[1]	917,650	897,479
Gross Unrealized Gains	[1]	4,191	14,491
Gross Unrealized Losses	[1]	(17,514)	(3,737)
Amortized Cost	[1]	930,973	886,725
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total available for sale investments		7,264	6,480
Gross Unrealized Gains			3
Gross Unrealized Losses		(4)
Amortized Cost		$ 7,268	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.