Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,401,341	$ 2,191,827
Total investments and cash	2,700,182	2,574,586
Gross Unrealized Gains	87,569	69,554
Gross Unrealized Losses	(11,734)	(33,530)
Amortized Cost	2,624,347	2,538,562
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	226,237	296,250
Amortized Cost	226,237	296,250
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	72,604	86,509
Amortized Cost	72,604	86,509
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,220,855	2,047,873
Gross Unrealized Gains	54,508	43,854
Gross Unrealized Losses	(10,747)	(32,980)
Amortized Cost	2,177,094	2,036,999
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	421,891	441,685
Gross Unrealized Gains	4,313	2,854
Gross Unrealized Losses	(4,339)	(8,855)
Amortized Cost	421,917	447,686
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	564,782	460,422
Gross Unrealized Gains	15,833	9,298
Gross Unrealized Losses	(2,883)	(13,651)
Amortized Cost	551,832	464,775
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	256,407	301,274
Gross Unrealized Gains	8,289	6,779
Gross Unrealized Losses	(1,753)	(6,016)
Amortized Cost	249,871	300,511
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	60,867	41,755
Gross Unrealized Gains	1,832	1,212
Gross Unrealized Losses	(101)	(161)
Amortized Cost	59,136	40,704
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	162,529	125,133
Gross Unrealized Gains	598	653
Gross Unrealized Losses	(381)	(480)
Amortized Cost	162,312	124,960
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	184,946	172,750
Gross Unrealized Gains	8,071	7,656
Gross Unrealized Losses	(131)	(374)
Amortized Cost	177,006	165,468
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	664,749	640,912
Gross Unrealized Gains	18,790	16,300
Gross Unrealized Losses	(2,366)	(7,031)
Amortized Cost	648,325	631,643
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	569,433	504,854
Gross Unrealized Gains	15,572	15,402
Gross Unrealized Losses	(1,159)	(3,443)
Amortized Cost	555,020	492,895
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments	144,977	143,954
Gross Unrealized Gains	31,262	25,700
Gross Unrealized Losses	(964)	(550)
Amortized Cost	114,679	118,804
Equity Securities | Preferred Stock
Gain (Loss) on Investments [Line Items]
Total investments	35,509
Gross Unrealized Gains	1,799
Gross Unrealized Losses	(23)
Amortized Cost	$ 33,733