Summary of Securities with Gross Unrealized Losses (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 357,076		$ 115,757
Less than 12 months, Gross Unrealized Losses	(7,224)		(2,879)
12 months or longer, Fair Value	7,668	[1]	12,661	[1]
12 months or longer, Gross Unrealized Losses	(406)	[1]	(844)	[1]
Total, Fair Value	364,744		128,418
Total, Gross Unrealized Losses	(7,630)		(3,723)
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses	(1,606)
Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses	(2,358)
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses	(503)
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Gross Unrealized Losses	(980)
Corporate bonds and loans
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Gross Unrealized Losses	(101)
Total, Gross Unrealized Losses	(930)
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	16,046		30,153
Less than 12 months, Gross Unrealized Losses	(881)		(2,284)
12 months or longer, Fair Value	1,299	[1]	3,950	[1]
12 months or longer, Gross Unrealized Losses	(239)	[1]	(667)	[1]
Total, Fair Value	17,345		34,103
Total, Gross Unrealized Losses	(1,120)		(2,951)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	341,030		85,604
Less than 12 months, Gross Unrealized Losses	(6,343)		(595)
12 months or longer, Fair Value	6,369	[1]	8,711	[1]
12 months or longer, Gross Unrealized Losses	(167)	[1]	(177)	[1]
Total, Fair Value	347,399		94,315
Total, Gross Unrealized Losses	(6,510)		(772)
Fixed Maturities | U.S. Treasury And Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	6,272		2,002
Less than 12 months, Gross Unrealized Losses	(133)		(1)
Total, Fair Value	6,272		2,002
Total, Gross Unrealized Losses	(133)		(1)
Fixed Maturities | Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	65,819		33,204
Less than 12 months, Gross Unrealized Losses	(1,543)		(132)
12 months or longer, Fair Value	4,550	[1]
12 months or longer, Gross Unrealized Losses	(63)	[1]
Total, Fair Value	70,369		33,204
Total, Gross Unrealized Losses	(1,606)		(132)
Fixed Maturities | Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	117,001		33,635
Less than 12 months, Gross Unrealized Losses	(2,356)		(172)
12 months or longer, Fair Value	188	[1]	640	[1]
12 months or longer, Gross Unrealized Losses	(2)	[1]	(17)	[1]
Total, Fair Value	117,189		34,275
Total, Gross Unrealized Losses	(2,358)		(189)
Fixed Maturities | Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	40,810		5,722
Less than 12 months, Gross Unrealized Losses	(502)		(3)
12 months or longer, Fair Value	3	[1]	4,763	[1]
12 months or longer, Gross Unrealized Losses	(1)	[1]	(6)	[1]
Total, Fair Value	40,813		10,485
Total, Gross Unrealized Losses	(503)		(9)
Fixed Maturities | Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	52,096		2,839
Less than 12 months, Gross Unrealized Losses	(980)		(13)
Total, Fair Value	52,096		2,839
Total, Gross Unrealized Losses	(980)		(13)
Fixed Maturities | Corporate bonds and loans
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	59,032		8,202
Less than 12 months, Gross Unrealized Losses	(829)		(274)
12 months or longer, Fair Value	1,628	[1]	3,308	[1]
12 months or longer, Gross Unrealized Losses	(101)	[1]	(154)	[1]
Total, Fair Value	60,660		11,510
Total, Gross Unrealized Losses	$ (930)		$ (428)

[1] Fixed maturities in a gross unrealized loss position for twelve months or longer are primarily comprised of non-credit losses on investment grade securities where management does not intend to sell, and it is more likely than not that the Company will not be forced to sell the security before recovery. The Company has analyzed these securities and has determined that they are not other than temporarily impaired.