Schedule of Amortized Cost and Estimated Fair Value of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,382,175		$ 1,357,322
Gross Unrealized Gains	87,326		75,930
Gross Unrealized Losses	(7,578)		(3,723)
Estimated Fair Value	1,461,923		1,429,529
Other than temporary impairments recognized in AOCI	(24)	[1]	(32)	[1]
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	191,425		167,179
Gross Unrealized Gains	63,281		32,847
Gross Unrealized Losses	(636)		(2,951)
Estimated Fair Value	254,070		197,075
Other Invested Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,065		3,049
Gross Unrealized Gains	424		83
Estimated Fair Value	3,489		3,132
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,187,685		1,187,094
Gross Unrealized Gains	23,621		43,000
Gross Unrealized Losses	(6,942)		(772)
Estimated Fair Value	1,204,364		1,229,322
Other than temporary impairments recognized in AOCI	(24)	[1]	(32)	[1]
Fixed Maturities | U.S. Treasury And Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,510		102,186
Gross Unrealized Gains	3,330		6,559
Gross Unrealized Losses	(166)		(1)
Estimated Fair Value	81,674		108,744
Fixed Maturities | Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	178,705		194,326
Gross Unrealized Gains	4,472		6,883
Gross Unrealized Losses	(2,241)		(132)
Estimated Fair Value	180,936		201,077
Fixed Maturities | Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	228,550		247,639
Gross Unrealized Gains	4,219		8,492
Gross Unrealized Losses	(2,859)		(189)
Estimated Fair Value	229,910		255,942
Other than temporary impairments recognized in AOCI	(5)	[1]	(8)	[1]
Fixed Maturities | Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	167,454		111,289
Gross Unrealized Gains	1,210		2,071
Gross Unrealized Losses	(228)		(9)
Estimated Fair Value	168,436		113,351
Other than temporary impairments recognized in AOCI	(19)	[1]	(24)	[1]
Fixed Maturities | Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	54,822		8,070
Gross Unrealized Gains	9		60
Gross Unrealized Losses	(856)		(13)
Estimated Fair Value	53,975		8,117
Fixed Maturities | Corporate bonds and loans
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	426,872		469,860
Gross Unrealized Gains	9,112		16,739
Gross Unrealized Losses	(592)		(428)
Estimated Fair Value	435,392		486,171
Fixed Maturities | Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,772		53,724
Gross Unrealized Gains	1,269		2,196
Estimated Fair Value	$ 54,041		$ 55,920

[1] Represents the total amount of other than temporary impairment losses relating to factors other than credit losses recognized in accumulated other comprehensive income ("AOCI").