Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,326,240	$ 2,191,827
Total investments and cash	2,589,124	2,574,586
Gross Unrealized Gains	75,381	69,554
Gross Unrealized Losses	(19,991)	(33,530)
Amortized Cost	2,533,734	2,538,562
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	207,843	296,250
Amortized Cost	207,843	296,250
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	55,041	86,509
Amortized Cost	55,041	86,509
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,162,713	2,047,873
Gross Unrealized Gains	47,940	43,854
Gross Unrealized Losses	(18,719)	(32,980)
Amortized Cost	2,133,492	2,036,999
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	417,942	441,685
Gross Unrealized Gains	3,066	2,854
Gross Unrealized Losses	(4,956)	(8,855)
Amortized Cost	419,832	447,686
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	491,598	460,422
Gross Unrealized Gains	12,216	9,298
Gross Unrealized Losses	(6,948)	(13,651)
Amortized Cost	486,330	464,775
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	265,818	301,274
Gross Unrealized Gains	7,182	6,779
Gross Unrealized Losses	(3,525)	(6,016)
Amortized Cost	262,161	300,511
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	57,852	41,755
Gross Unrealized Gains	1,573	1,212
Gross Unrealized Losses	(136)	(161)
Amortized Cost	56,415	40,704
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	162,226	125,133
Gross Unrealized Gains	639	653
Gross Unrealized Losses	(563)	(480)
Amortized Cost	162,150	124,960
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	173,333	172,750
Gross Unrealized Gains	7,792	7,656
Gross Unrealized Losses	(272)	(374)
Amortized Cost	165,813	165,468
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	659,229	640,912
Gross Unrealized Gains	17,186	16,300
Gross Unrealized Losses	(4,496)	(7,031)
Amortized Cost	646,539	631,643
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	593,944	504,854
Gross Unrealized Gains	15,472	15,402
Gross Unrealized Losses	(2,319)	(3,443)
Amortized Cost	580,791	492,895
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments	141,476	143,954
Gross Unrealized Gains	26,589	25,700
Gross Unrealized Losses	(1,271)	(550)
Amortized Cost	116,158	118,804
Equity Securities | Preferred Stock
Gain (Loss) on Investments [Line Items]
Total investments	22,051
Gross Unrealized Gains	852
Gross Unrealized Losses	(1)
Amortized Cost	$ 21,200