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, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 1,010,970	$ 468,937
Less than 12 months Gross Unrealized Losses	(20,517)	(4,029)
Greater than 12 months Fair Value	156,975	282,337
Greater than 12 months Gross Unrealized Losses	(8,881)	(7,622)
Fair Value	1,167,945	751,274
Gross Unrealized Losses	(29,398)	(11,651)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	952,439	449,247
Less than 12 months Gross Unrealized Losses	(17,504)	(2,732)
Greater than 12 months Fair Value	156,975	282,099
Greater than 12 months Gross Unrealized Losses	(8,881)	(7,615)
Fair Value	1,109,414	731,346
Gross Unrealized Losses	(26,385)	(10,347)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	142,233	87,915
Less than 12 months Gross Unrealized Losses	(3,032)	(1,061)
Greater than 12 months Fair Value	22,230	117,683
Greater than 12 months Gross Unrealized Losses	(6,182)	(4,904)
Fair Value	164,463	205,598
Gross Unrealized Losses	(9,214)	(5,965)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	50,577	16,349
Less than 12 months Gross Unrealized Losses	(549)	(60)
Greater than 12 months Fair Value	4,808	37,340
Greater than 12 months Gross Unrealized Losses	(232)	(498)
Fair Value	55,385	53,689
Gross Unrealized Losses	(781)	(558)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	164,817	18,881
Less than 12 months Gross Unrealized Losses	(1,315)	(80)
Greater than 12 months Fair Value	29,862	58,301
Greater than 12 months Gross Unrealized Losses	(767)	(918)
Fair Value	194,679	77,182
Gross Unrealized Losses	(2,082)	(998)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	3,910	5,625
Less than 12 months Gross Unrealized Losses	(5)	(50)
Greater than 12 months Fair Value	1,684	1,728
Greater than 12 months Gross Unrealized Losses	(77)	(88)
Fair Value	5,594	7,353
Gross Unrealized Losses	(82)	(138)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	112,479	110,275
Less than 12 months Gross Unrealized Losses	(663)	(539)
Greater than 12 months Fair Value	81,477	34,530
Greater than 12 months Gross Unrealized Losses	(961)	(425)
Fair Value	193,956	144,805
Gross Unrealized Losses	(1,624)	(964)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	83,024	19,741
Less than 12 months Gross Unrealized Losses	(1,826)	(71)
Greater than 12 months Fair Value	3,065	1,391
Greater than 12 months Gross Unrealized Losses	(38)	(27)
Fair Value	86,089	21,132
Gross Unrealized Losses	(1,864)	(98)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	364,230	154,522
Less than 12 months Gross Unrealized Losses	(3,809)	(740)
Greater than 12 months Fair Value	116,088	95,950
Greater than 12 months Gross Unrealized Losses	(1,843)	(1,458)
Fair Value	480,318	250,472
Gross Unrealized Losses	(5,652)	(2,198)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	395,399	190,461
Less than 12 months Gross Unrealized Losses	(10,114)	(871)
Greater than 12 months Fair Value	13,849	31,126
Greater than 12 months Gross Unrealized Losses	(624)	(755)
Fair Value	409,248	221,587
Gross Unrealized Losses	(10,738)	(1,626)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	58,531	19,690
Less than 12 months Gross Unrealized Losses	(3,013)	(1,297)
Greater than 12 months Fair Value		238
Greater than 12 months Gross Unrealized Losses		(7)
Fair Value	58,531	19,928
Gross Unrealized Losses	$ (3,013)	$ (1,304)