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	Mar. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 1,082,035	$ 1,256,887
Less than 12 months Gross Unrealized Losses	(18,519)	(24,493)
Greater than 12 months Fair Value	91,351	127,403
Greater than 12 months Gross Unrealized Losses	(3,818)	(5,706)
Fair Value	1,173,386	1,384,290
Gross Unrealized Losses	(22,337)	(30,199)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	1,010,979	1,111,873
Less than 12 months Gross Unrealized Losses	(16,739)	(19,074)
Greater than 12 months Fair Value	88,794	125,409
Greater than 12 months Gross Unrealized Losses	(3,726)	(5,340)
Fair Value	1,099,773	1,237,282
Gross Unrealized Losses	(20,465)	(24,414)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	153,332	150,891
Less than 12 months Gross Unrealized Losses	(2,275)	(2,570)
Greater than 12 months Fair Value	11,844	16,819
Greater than 12 months Gross Unrealized Losses	(1,396)	(2,558)
Fair Value	165,176	167,710
Gross Unrealized Losses	(3,671)	(5,128)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	120,372	137,731
Less than 12 months Gross Unrealized Losses	(2,534)	(3,111)
Greater than 12 months Fair Value	17,706	13,255
Greater than 12 months Gross Unrealized Losses	(819)	(925)
Fair Value	138,078	150,986
Gross Unrealized Losses	(3,353)	(4,036)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	347,179	349,119
Less than 12 months Gross Unrealized Losses	(6,622)	(6,155)
Greater than 12 months Fair Value	11,810	12,401
Greater than 12 months Gross Unrealized Losses	(429)	(430)
Fair Value	358,989	361,520
Gross Unrealized Losses	(7,051)	(6,585)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	486	953
Less than 12 months Gross Unrealized Losses	(5)	(18)
Greater than 12 months Fair Value	1,204	926
Greater than 12 months Gross Unrealized Losses	(32)	(37)
Fair Value	1,690	1,879
Gross Unrealized Losses	(37)	(55)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	68,847	95,514
Less than 12 months Gross Unrealized Losses	(708)	(970)
Greater than 12 months Fair Value	17,944	48,093
Greater than 12 months Gross Unrealized Losses	(61)	(208)
Fair Value	86,791	143,607
Gross Unrealized Losses	(769)	(1,178)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	51,336	51,932
Less than 12 months Gross Unrealized Losses	(1,024)	(1,164)
Greater than 12 months Fair Value	6,385	7,910
Greater than 12 months Gross Unrealized Losses	(819)	(740)
Fair Value	57,721	59,842
Gross Unrealized Losses	(1,843)	(1,904)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	467,848	497,518
Less than 12 months Gross Unrealized Losses	(8,359)	(8,307)
Greater than 12 months Fair Value	37,343	69,330
Greater than 12 months Gross Unrealized Losses	(1,341)	(1,415)
Fair Value	505,191	566,848
Gross Unrealized Losses	(9,700)	(9,722)
Fixed maturities | Corporate exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	269,427	325,733
Less than 12 months Gross Unrealized Losses	(3,571)	(5,086)
Greater than 12 months Fair Value	21,901	26,005
Greater than 12 months Gross Unrealized Losses	(170)	(442)
Fair Value	291,328	351,738
Gross Unrealized Losses	(3,741)	(5,528)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	71,056	145,014
Less than 12 months Gross Unrealized Losses	(1,780)	(5,419)
Greater than 12 months Fair Value	2,557	1,994
Greater than 12 months Gross Unrealized Losses	(92)	(366)
Fair Value	73,613	147,008
Gross Unrealized Losses	(1,872)	(5,785)
Equity Securities | Common stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	28,586	31,272
Less than 12 months Gross Unrealized Losses	(652)	(634)
Greater than 12 months Fair Value	0	1,471
Greater than 12 months Gross Unrealized Losses	0	(330)
Fair Value	28,586	32,743
Gross Unrealized Losses	(652)	(964)
Equity Securities | Preferred stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	42,470	113,742
Less than 12 months Gross Unrealized Losses	(1,128)	(4,785)
Greater than 12 months Fair Value	2,557	523
Greater than 12 months Gross Unrealized Losses	(92)	(36)
Fair Value	45,027	114,265
Gross Unrealized Losses	$ (1,220)	$ (4,821)