Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,473,668	$ 2,191,827
Total investments and cash	2,803,521	2,574,586
Gross Unrealized Gains	74,082	69,554
Gross Unrealized Losses	(14,549)	(33,530)
Amortized Cost	2,743,988	2,538,562
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	255,445	296,250
Gross Unrealized Gains	11
Amortized Cost	255,434	296,250
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	74,408	86,509
Amortized Cost	74,408	86,509
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,310,195	2,047,873
Gross Unrealized Gains	46,339	43,854
Gross Unrealized Losses	(13,528)	(32,980)
Amortized Cost	2,277,384	2,036,999
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	414,348	441,685
Gross Unrealized Gains	2,712	2,854
Gross Unrealized Losses	(6,215)	(8,855)
Amortized Cost	417,851	447,686
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	550,327	460,422
Gross Unrealized Gains	16,585	9,298
Gross Unrealized Losses	(1,360)	(13,651)
Amortized Cost	535,102	464,775
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	296,198	301,274
Gross Unrealized Gains	7,201	6,779
Gross Unrealized Losses	(2,363)	(6,016)
Amortized Cost	291,360	300,511
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	34,738	41,755
Gross Unrealized Gains	1,238	1,212
Gross Unrealized Losses	(133)	(161)
Amortized Cost	33,633	40,704
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	195,180	125,133
Gross Unrealized Gains	414	653
Gross Unrealized Losses	(519)	(480)
Amortized Cost	195,285	124,960
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	210,712	172,750
Gross Unrealized Gains	6,244	7,656
Gross Unrealized Losses	(588)	(374)
Amortized Cost	205,056	165,468
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	736,828	640,912
Gross Unrealized Gains	15,097	16,300
Gross Unrealized Losses	(3,603)	(7,031)
Amortized Cost	725,334	631,643
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	608,692	504,854
Gross Unrealized Gains	11,945	15,402
Gross Unrealized Losses	(2,350)	(3,443)
Amortized Cost	599,097	492,895
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments	113,250	143,954
Gross Unrealized Gains	26,251	25,700
Gross Unrealized Losses	(772)	(550)
Amortized Cost	87,771	118,804
Equity Securities | Preferred Stock
Gain (Loss) on Investments [Line Items]
Total investments	50,223
Gross Unrealized Gains	1,481
Gross Unrealized Losses	(249)
Amortized Cost	$ 48,991