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, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 252,514	$ 468,937
Less than 12 months Gross Unrealized Losses	(4,338)	(4,029)
Greater than 12 months Fair Value	103,478	282,337
Greater than 12 months Gross Unrealized Losses	(7,972)	(7,622)
Fair Value	355,992	751,274
Gross Unrealized Losses	(12,310)	(11,651)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	241,744	449,247
Less than 12 months Gross Unrealized Losses	(3,710)	(2,732)
Greater than 12 months Fair Value	101,675	282,099
Greater than 12 months Gross Unrealized Losses	(7,563)	(7,615)
Fair Value	343,419	731,346
Gross Unrealized Losses	(11,273)	(10,347)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	38,187	87,915
Less than 12 months Gross Unrealized Losses	(2,879)	(1,061)
Greater than 12 months Fair Value	27,210	117,683
Greater than 12 months Gross Unrealized Losses	(5,853)	(4,904)
Fair Value	65,397	205,598
Gross Unrealized Losses	(8,732)	(5,965)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	23,698	16,349
Less than 12 months Gross Unrealized Losses	(127)	(60)
Greater than 12 months Fair Value	3,514	37,340
Greater than 12 months Gross Unrealized Losses	(194)	(498)
Fair Value	27,212	53,689
Gross Unrealized Losses	(321)	(558)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	25,824	18,881
Less than 12 months Gross Unrealized Losses	(78)	(80)
Greater than 12 months Fair Value	22,530	58,301
Greater than 12 months Gross Unrealized Losses	(511)	(918)
Fair Value	48,354	77,182
Gross Unrealized Losses	(589)	(998)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	2,002	5,625
Less than 12 months Gross Unrealized Losses	(22)	(50)
Greater than 12 months Fair Value	1,623	1,728
Greater than 12 months Gross Unrealized Losses	(95)	(88)
Fair Value	3,625	7,353
Gross Unrealized Losses	(117)	(138)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	61,801	110,275
Less than 12 months Gross Unrealized Losses	(209)	(539)
Greater than 12 months Fair Value	41,475	34,530
Greater than 12 months Gross Unrealized Losses	(345)	(425)
Fair Value	103,276	144,805
Gross Unrealized Losses	(554)	(964)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	4,088	19,741
Less than 12 months Gross Unrealized Losses	(9)	(71)
Greater than 12 months Fair Value	1,369	1,391
Greater than 12 months Gross Unrealized Losses	(19)	(27)
Fair Value	5,457	21,132
Gross Unrealized Losses	(28)	(98)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	86,144	190,461
Less than 12 months Gross Unrealized Losses	(386)	(871)
Greater than 12 months Fair Value	3,954	31,126
Greater than 12 months Gross Unrealized Losses	(546)	(755)
Fair Value	90,098	221,587
Gross Unrealized Losses	(932)	(1,626)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	93,715	154,522
Less than 12 months Gross Unrealized Losses	(318)	(740)
Greater than 12 months Fair Value	66,997	95,950
Greater than 12 months Gross Unrealized Losses	(970)	(1,458)
Fair Value	160,712	250,472
Gross Unrealized Losses	(1,288)	(2,198)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	10,770	19,690
Less than 12 months Gross Unrealized Losses	(628)	(1,297)
Greater than 12 months Fair Value	1,803	238
Greater than 12 months Gross Unrealized Losses	(409)	(7)
Fair Value	12,573	19,928
Gross Unrealized Losses	$ (1,037)	$ (1,304)