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	Jun. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 885,904	$ 1,256,887
Less than 12 months Gross Unrealized Losses	(12,553)	(24,493)
Greater than 12 months Fair Value	81,623	127,403
Greater than 12 months Gross Unrealized Losses	(3,312)	(5,706)
Fair Value	967,527	1,384,290
Gross Unrealized Losses	(15,865)	(30,199)
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	837,702	1,111,873
Less than 12 months Gross Unrealized Losses	(11,063)	(19,074)
Greater than 12 months Fair Value	81,095	125,409
Greater than 12 months Gross Unrealized Losses	(3,280)	(5,340)
Fair Value	918,797	1,237,282
Gross Unrealized Losses	(14,343)	(24,414)
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds, and Foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	127,900	150,891
Less than 12 months Gross Unrealized Losses	(1,339)	(2,570)
Greater than 12 months Fair Value	17,878	16,819
Greater than 12 months Gross Unrealized Losses	(1,321)	(2,558)
Fair Value	145,778	167,710
Gross Unrealized Losses	(2,660)	(5,128)
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	75,488	137,731
Less than 12 months Gross Unrealized Losses	(1,405)	(3,111)
Greater than 12 months Fair Value	21,502	13,255
Greater than 12 months Gross Unrealized Losses	(599)	(925)
Fair Value	96,990	150,986
Gross Unrealized Losses	(2,004)	(4,036)
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	307,616	349,119
Less than 12 months Gross Unrealized Losses	(4,995)	(6,155)
Greater than 12 months Fair Value	11,068	12,401
Greater than 12 months Gross Unrealized Losses	(394)	(430)
Fair Value	318,684	361,520
Gross Unrealized Losses	(5,389)	(6,585)
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	262	953
Less than 12 months Gross Unrealized Losses	(1)	(18)
Greater than 12 months Fair Value	746	926
Greater than 12 months Gross Unrealized Losses	(24)	(37)
Fair Value	1,008	1,879
Gross Unrealized Losses	(25)	(55)
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	77,936	95,514
Less than 12 months Gross Unrealized Losses	(465)	(970)
Greater than 12 months Fair Value	2,694	48,093
Greater than 12 months Gross Unrealized Losses	(13)	(208)
Fair Value	80,630	143,607
Gross Unrealized Losses	(478)	(1,178)
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	33,780	51,932
Less than 12 months Gross Unrealized Losses	(651)	(1,164)
Greater than 12 months Fair Value	6,359	7,910
Greater than 12 months Gross Unrealized Losses	(781)	(740)
Fair Value	40,139	59,842
Gross Unrealized Losses	(1,432)	(1,904)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	419,594	497,518
Less than 12 months Gross Unrealized Losses	(6,112)	(8,307)
Greater than 12 months Fair Value	20,867	69,330
Greater than 12 months Gross Unrealized Losses	(1,212)	(1,415)
Fair Value	440,461	566,848
Gross Unrealized Losses	(7,324)	(9,722)
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	214,720	325,733
Less than 12 months Gross Unrealized Losses	(2,207)	(5,086)
Greater than 12 months Fair Value	20,848	26,005
Greater than 12 months Gross Unrealized Losses	(148)	(442)
Fair Value	235,568	351,738
Gross Unrealized Losses	(2,355)	(5,528)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	48,202	145,014
Less than 12 months Gross Unrealized Losses	(1,490)	(5,419)
Greater than 12 months Fair Value	528	1,994
Greater than 12 months Gross Unrealized Losses	(32)	(366)
Fair Value	48,730	147,008
Gross Unrealized Losses	(1,522)	(5,785)
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	26,598	31,272
Less than 12 months Gross Unrealized Losses	(1,023)	(634)
Greater than 12 months Fair Value	0	1,471
Greater than 12 months Gross Unrealized Losses	0	(330)
Fair Value	26,598	32,743
Gross Unrealized Losses	(1,023)	(964)
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	21,604	113,742
Less than 12 months Gross Unrealized Losses	(467)	(4,785)
Greater than 12 months Fair Value	528	523
Greater than 12 months Gross Unrealized Losses	(32)	(36)
Fair Value	22,132	114,265
Gross Unrealized Losses	$ (499)	$ (4,821)