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	Sep. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 702,989	$ 1,256,887
Less than 12 months Gross Unrealized Losses	(7,244)	(24,493)
Greater than 12 months Fair Value	227,378	127,403
Greater than 12 months Gross Unrealized Losses	(4,346)	(5,706)
Fair Value	930,367	1,384,290
Gross Unrealized Losses	(11,590)	(30,199)
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	655,581	1,111,873
Less than 12 months Gross Unrealized Losses	(5,816)	(19,074)
Greater than 12 months Fair Value	226,624	125,409
Greater than 12 months Gross Unrealized Losses	(4,314)	(5,340)
Fair Value	882,205	1,237,282
Gross Unrealized Losses	(10,130)	(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	67,562	150,891
Less than 12 months Gross Unrealized Losses	(344)	(2,570)
Greater than 12 months Fair Value	31,928	16,819
Greater than 12 months Gross Unrealized Losses	(263)	(2,558)
Fair Value	99,490	167,710
Gross Unrealized Losses	(607)	(5,128)
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	80,570	137,731
Less than 12 months Gross Unrealized Losses	(732)	(3,111)
Greater than 12 months Fair Value	31,125	13,255
Greater than 12 months Gross Unrealized Losses	(749)	(925)
Fair Value	111,695	150,986
Gross Unrealized Losses	(1,481)	(4,036)
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	237,804	349,119
Less than 12 months Gross Unrealized Losses	(2,846)	(6,155)
Greater than 12 months Fair Value	56,656	12,401
Greater than 12 months Gross Unrealized Losses	(1,452)	(430)
Fair Value	294,460	361,520
Gross Unrealized Losses	(4,298)	(6,585)
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	7,899	953
Less than 12 months Gross Unrealized Losses	(6)	(18)
Greater than 12 months Fair Value	586	926
Greater than 12 months Gross Unrealized Losses	(21)	(37)
Fair Value	8,485	1,879
Gross Unrealized Losses	(27)	(55)
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	69,885	95,514
Less than 12 months Gross Unrealized Losses	(379)	(970)
Greater than 12 months Fair Value	4,942	48,093
Greater than 12 months Gross Unrealized Losses	(47)	(208)
Fair Value	74,827	143,607
Gross Unrealized Losses	(426)	(1,178)
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	41,572	51,932
Less than 12 months Gross Unrealized Losses	(594)	(1,164)
Greater than 12 months Fair Value	7,924	7,910
Greater than 12 months Gross Unrealized Losses	(714)	(740)
Fair Value	49,496	59,842
Gross Unrealized Losses	(1,308)	(1,904)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	357,160	497,518
Less than 12 months Gross Unrealized Losses	(3,825)	(8,307)
Greater than 12 months Fair Value	70,108	69,330
Greater than 12 months Gross Unrealized Losses	(2,234)	(1,415)
Fair Value	427,268	566,848
Gross Unrealized Losses	(6,059)	(9,722)
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	150,289	325,733
Less than 12 months Gross Unrealized Losses	(915)	(5,086)
Greater than 12 months Fair Value	93,463	26,005
Greater than 12 months Gross Unrealized Losses	(1,068)	(442)
Fair Value	243,752	351,738
Gross Unrealized Losses	(1,983)	(5,528)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	47,408	145,014
Less than 12 months Gross Unrealized Losses	(1,428)	(5,419)
Greater than 12 months Fair Value	754	1,994
Greater than 12 months Gross Unrealized Losses	(32)	(366)
Fair Value	48,162	147,008
Gross Unrealized Losses	(1,460)	(5,785)
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	15,633	31,272
Less than 12 months Gross Unrealized Losses	(814)	(634)
Greater than 12 months Fair Value	87	1,471
Greater than 12 months Gross Unrealized Losses	(1)	(330)
Fair Value	15,720	32,743
Gross Unrealized Losses	(815)	(964)
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	31,775	113,742
Less than 12 months Gross Unrealized Losses	(614)	(4,785)
Greater than 12 months Fair Value	667	523
Greater than 12 months Gross Unrealized Losses	(31)	(36)
Fair Value	32,442	114,265
Gross Unrealized Losses	$ (645)	$ (4,821)