Cash And Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,414,685	$ 2,233,498
Gross Unrealized Gains	142,138	105,381
Gross Unrealized Losses	(1,515)	(11,740)
Amortized Cost	2,274,062	2,139,857
OTTI Recognized in OCI	(278)	(1,682)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	2,085,058	1,888,069
Gross Unrealized Gains	110,761	82,141
Gross Unrealized Losses	(1,237)	(10,782)
Amortized Cost	1,975,534	1,816,710
OTTI Recognized in OCI	(278)	(1,682)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	465,215	336,070
Gross Unrealized Gains	11,881	8,979
Gross Unrealized Losses	(55)	(383)
Amortized Cost	453,389	327,474
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	443,994	410,836
Gross Unrealized Gains	34,451	28,887
Gross Unrealized Losses	(66)	(108)
Amortized Cost	409,609	382,057
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	405,229	395,860
Gross Unrealized Gains	17,582	17,321
Gross Unrealized Losses	(56)	(3)
Amortized Cost	387,703	378,542
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	39,095	23,148
Gross Unrealized Gains	711	8
Gross Unrealized Losses	(837)	(2,848)
Amortized Cost	39,221	25,988
OTTI Recognized in OCI	(278)	(1,682)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	50,897	48,934
Gross Unrealized Gains	1,069	695
Gross Unrealized Losses	(72)	(75)
Amortized Cost	49,900	48,314
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	208,742	216,034
Gross Unrealized Gains	18,126	10,508
Gross Unrealized Losses	(50)	(593)
Amortized Cost	190,666	206,119
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	703,963	683,976
Gross Unrealized Gains	37,488	28,532
Gross Unrealized Losses	(1,015)	(3,519)
Amortized Cost	667,490	658,963
OTTI Recognized in OCI	(278)	(1,682)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	471,886	457,187
Gross Unrealized Gains	26,941	15,743
Gross Unrealized Losses	(101)	(6,772)
Amortized Cost	445,046	448,216
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	105,930	95,849
Gross Unrealized Gains	31,377	23,240
Gross Unrealized Losses	(278)	(958)
Amortized Cost	74,831	73,567
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	187,445	122,220
Amortized Cost	187,445	122,220
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	36,252	127,360
Amortized Cost	$ 36,252	$ 127,360