Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		$ 911,581	$ 1,256,887
Less than 12 months Gross Unrealized Losses		(6,271)	(24,493)
Greater than 12 months Fair Value		508,338	127,403
Greater than 12 months Gross Unrealized Losses		(9,913)	(5,706)
Fair Value		1,419,919	1,384,290
Gross Unrealized Losses		(16,184)	(30,199)
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		849,475	1,111,873
Less than 12 months Gross Unrealized Losses		(4,666)	(19,074)
Greater than 12 months Fair Value		505,906	125,409
Greater than 12 months Gross Unrealized Losses		(9,846)	(5,340)
Fair Value		1,355,381	1,237,282
Gross Unrealized Losses		(14,512)	(24,414)
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		273,672	150,891
Less than 12 months Gross Unrealized Losses		(1,502)	(2,570)
Greater than 12 months Fair Value		54,484	16,819
Greater than 12 months Gross Unrealized Losses		(512)	(2,558)
Fair Value		328,156	167,710
Gross Unrealized Losses		(2,014)	(5,128)
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		74,097	137,731
Less than 12 months Gross Unrealized Losses		(503)	(3,111)
Greater than 12 months Fair Value		45,085	13,255
Greater than 12 months Gross Unrealized Losses		(920)	(925)
Fair Value		119,182	150,986
Gross Unrealized Losses		(1,423)	(4,036)
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		87,496	349,119
Less than 12 months Gross Unrealized Losses		(346)	(6,155)
Greater than 12 months Fair Value		236,745	12,401
Greater than 12 months Gross Unrealized Losses		(5,068)	(430)
Fair Value		324,241	361,520
Gross Unrealized Losses		(5,414)	(6,585)
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		12,418	953
Less than 12 months Gross Unrealized Losses		(62)	(18)
Greater than 12 months Fair Value		546	926
Greater than 12 months Gross Unrealized Losses		(17)	(37)
Fair Value		12,964	1,879
Gross Unrealized Losses		(79)	(55)
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		85,877	95,514
Less than 12 months Gross Unrealized Losses		(468)	(970)
Greater than 12 months Fair Value		24,733	48,093
Greater than 12 months Gross Unrealized Losses		(195)	(208)
Fair Value		110,610	143,607
Gross Unrealized Losses		(663)	(1,178)
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		20,482	51,932
Less than 12 months Gross Unrealized Losses		(95)	(1,164)
Greater than 12 months Fair Value		22,903	7,910
Greater than 12 months Gross Unrealized Losses		(1,087)	(740)
Fair Value		43,385	59,842
Gross Unrealized Losses		(1,182)	(1,904)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		206,273	497,518
Less than 12 months Gross Unrealized Losses		(971)	(8,307)
Greater than 12 months Fair Value		284,927	69,330
Greater than 12 months Gross Unrealized Losses		(6,367)	(1,415)
Fair Value		491,200	566,848
Gross Unrealized Losses		(7,338)	(9,722)
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	[1]	295,433	325,733
Less than 12 months Gross Unrealized Losses	[1]	(1,690)	(5,086)
Greater than 12 months Fair Value	[1]	121,410	26,005
Greater than 12 months Gross Unrealized Losses	[1]	(2,047)	(442)
Fair Value	[1]	416,843	351,738
Gross Unrealized Losses	[1]	(3,737)	(5,528)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		62,106	145,014
Less than 12 months Gross Unrealized Losses		(1,605)	(5,419)
Greater than 12 months Fair Value		2,432	1,994
Greater than 12 months Gross Unrealized Losses		(67)	(366)
Fair Value		64,538	147,008
Gross Unrealized Losses		(1,672)	(5,785)
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		11,245	31,272
Less than 12 months Gross Unrealized Losses		(81)	(634)
Greater than 12 months Fair Value		1,770	1,471
Greater than 12 months Gross Unrealized Losses		(31)	(330)
Fair Value		13,015	32,743
Gross Unrealized Losses		(112)	(964)
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		50,861	113,742
Less than 12 months Gross Unrealized Losses		(1,524)	(4,785)
Greater than 12 months Fair Value		662	523
Greater than 12 months Gross Unrealized Losses		(36)	(36)
Fair Value		51,523	114,265
Gross Unrealized Losses		$ (1,560)	$ (4,821)

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