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	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 855,121	$ 468,937
Less than 12 months Gross Unrealized Losses	(13,609)	(4,029)
Greater than 12 months Fair Value	102,040	282,337
Greater than 12 months Gross Unrealized Losses	(6,049)	(7,622)
Fair Value	957,161	751,274
Gross Unrealized Losses	(19,658)	(11,651)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	782,264	449,247
Less than 12 months Gross Unrealized Losses	(10,037)	(2,732)
Greater than 12 months Fair Value	102,040	282,099
Greater than 12 months Gross Unrealized Losses	(6,049)	(7,615)
Fair Value	884,304	731,346
Gross Unrealized Losses	(16,086)	(10,347)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	99,590	87,915
Less than 12 months Gross Unrealized Losses	(2,204)	(1,061)
Greater than 12 months Fair Value	21,152	117,683
Greater than 12 months Gross Unrealized Losses	(3,969)	(4,904)
Fair Value	120,742	205,598
Gross Unrealized Losses	(6,173)	(5,965)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	168,021	16,349
Less than 12 months Gross Unrealized Losses	(2,186)	(60)
Greater than 12 months Fair Value	4,726	37,340
Greater than 12 months Gross Unrealized Losses	(345)	(498)
Fair Value	172,747	53,689
Gross Unrealized Losses	(2,531)	(558)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	117,372	18,881
Less than 12 months Gross Unrealized Losses	(1,215)	(80)
Greater than 12 months Fair Value	21,434	58,301
Greater than 12 months Gross Unrealized Losses	(885)	(918)
Fair Value	138,806	77,182
Gross Unrealized Losses	(2,100)	(998)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	5,361	5,625
Less than 12 months Gross Unrealized Losses	(29)	(50)
Greater than 12 months Fair Value	2,110	1,728
Greater than 12 months Gross Unrealized Losses	(73)	(88)
Fair Value	7,471	7,353
Gross Unrealized Losses	(102)	(138)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	69,998	110,275
Less than 12 months Gross Unrealized Losses	(161)	(539)
Greater than 12 months Fair Value	46,500	34,530
Greater than 12 months Gross Unrealized Losses	(327)	(425)
Fair Value	116,498	144,805
Gross Unrealized Losses	(488)	(964)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	70,869	19,741
Less than 12 months Gross Unrealized Losses	(1,459)	(71)
Greater than 12 months Fair Value	2,352	1,391
Greater than 12 months Gross Unrealized Losses	(22)	(27)
Fair Value	73,221	21,132
Gross Unrealized Losses	(1,481)	(98)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	251,053	190,461
Less than 12 months Gross Unrealized Losses	(2,783)	(871)
Greater than 12 months Fair Value	3,766	31,126
Greater than 12 months Gross Unrealized Losses	(428)	(755)
Fair Value	254,819	221,587
Gross Unrealized Losses	(3,211)	(1,626)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	263,600	154,522
Less than 12 months Gross Unrealized Losses	(2,864)	(740)
Greater than 12 months Fair Value	72,396	95,950
Greater than 12 months Gross Unrealized Losses	(1,307)	(1,458)
Fair Value	335,996	250,472
Gross Unrealized Losses	(4,171)	(2,198)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	72,857	19,690
Less than 12 months Gross Unrealized Losses	(3,572)	(1,297)
Greater than 12 months Fair Value		238
Greater than 12 months Gross Unrealized Losses		(7)
Fair Value	72,857	19,928
Gross Unrealized Losses	$ (3,572)	$ (1,304)