Cash And Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,294,448	$ 2,233,498
Gross Unrealized Gains	111,822	105,381
Gross Unrealized Losses	(4,414)	(11,740)
Amortized Cost	2,187,040	2,139,857
OTTI Recognized in OCI	(1,037)	(1,682)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	1,943,563	1,888,069
Gross Unrealized Gains	84,559	82,141
Gross Unrealized Losses	(4,072)	(10,782)
Amortized Cost	1,863,076	1,816,710
OTTI Recognized in OCI	(1,037)	(1,682)
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	418,876	336,070
Gross Unrealized Gains	8,032	8,979
Gross Unrealized Losses	(856)	(383)
Amortized Cost	411,700	327,474
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	398,399	410,836
Gross Unrealized Gains	28,185	28,887
Gross Unrealized Losses	(156)	(108)
Amortized Cost	370,370	382,057
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	385,239	395,860
Gross Unrealized Gains	16,240	17,321
Gross Unrealized Losses	(7)	(3)
Amortized Cost	369,006	378,542
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	20,254	23,148
Gross Unrealized Gains	35	8
Gross Unrealized Losses	(2,009)	(2,848)
Amortized Cost	22,228	25,988
OTTI Recognized in OCI	(1,037)	(1,682)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	49,077	48,934
Gross Unrealized Gains	874	695
Gross Unrealized Losses	(45)	(75)
Amortized Cost	48,248	48,314
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	212,609	216,034
Gross Unrealized Gains	13,248	10,508
Gross Unrealized Losses	(76)	(593)
Amortized Cost	199,437	206,119
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	667,179	683,976
Gross Unrealized Gains	30,397	28,532
Gross Unrealized Losses	(2,137)	(3,519)
Amortized Cost	638,919	658,963
OTTI Recognized in OCI	(1,037)	(1,682)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	459,109	457,187
Gross Unrealized Gains	17,945	15,743
Gross Unrealized Losses	(923)	(6,772)
Amortized Cost	442,087	448,216
Equity Securities | Common stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	102,400	95,849
Gross Unrealized Gains	27,263	23,240
Gross Unrealized Losses	(342)	(958)
Amortized Cost	75,479	73,567
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	202,977	122,220
Amortized Cost	202,977	122,220
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	45,508	127,360
Amortized Cost	$ 45,508	$ 127,360