Summary of Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		$ 510,011	$ 453,042
Less than 12 months, Gross Unrealized Losses		(8,194)	(5,160)
12 months or longer, Fair Value	[1]	36,136	115,363
12 months or longer, Gross Unrealized Losses	[1]	(340)	(1,739)
Total, Fair Value		546,147	568,405
Total, Gross Unrealized Losses		(8,534)	(6,899)
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(670)
Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(159)
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Gross Unrealized Losses		(28)
Total, Gross Unrealized Losses		(616)
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(827)
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(1,072)
Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(231)
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		38,128	20,002
Less than 12 months, Gross Unrealized Losses		(4,950)	(2,808)
12 months or longer, Fair Value	[1]		1,577
12 months or longer, Gross Unrealized Losses	[1]		(130)
Total, Fair Value		38,128	21,579
Total, Gross Unrealized Losses		(4,950)	(2,938)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		471,883	433,040
Less than 12 months, Gross Unrealized Losses		(3,244)	(2,352)
12 months or longer, Fair Value	[1]	36,136	113,786
12 months or longer, Gross Unrealized Losses	[1]	(340)	(1,609)
Total, Fair Value		508,019	546,826
Total, Gross Unrealized Losses		(3,584)	(3,961)
Fixed Maturities | U.S. Treasury and Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value			11,728
Less than 12 months, Gross Unrealized Losses			(9)
12 months or longer, Fair Value	[1]	3,413	3,343
12 months or longer, Gross Unrealized Losses	[1]	(9)	(74)
Total, Fair Value		3,413	15,071
Total, Gross Unrealized Losses		(9)	(83)
Fixed Maturities | Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		43,194	28,684
Less than 12 months, Gross Unrealized Losses		(573)	(314)
12 months or longer, Fair Value	[1]	9,510	28,061
12 months or longer, Gross Unrealized Losses	[1]	(97)	(383)
Total, Fair Value		52,704	56,745
Total, Gross Unrealized Losses		(670)	(697)
Fixed Maturities | Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		23,004	2,818
Less than 12 months, Gross Unrealized Losses		(114)	(7)
12 months or longer, Fair Value	[1]	255	51,203
12 months or longer, Gross Unrealized Losses	[1]	(45)	(757)
Total, Fair Value		23,259	54,021
Total, Gross Unrealized Losses		(159)	(764)
Fixed Maturities | Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		107,361	92,123
Less than 12 months, Gross Unrealized Losses		(588)	(283)
12 months or longer, Fair Value	[1]	4,531	1,683
12 months or longer, Gross Unrealized Losses	[1]	(28)	(20)
Total, Fair Value		111,892	93,806
Total, Gross Unrealized Losses		(616)	(303)
Fixed Maturities | Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		110,702	92,664
Less than 12 months, Gross Unrealized Losses		(687)	(525)
12 months or longer, Fair Value	[1]	15,453	26,280
12 months or longer, Gross Unrealized Losses	[1]	(140)	(322)
Total, Fair Value		126,155	118,944
Total, Gross Unrealized Losses		(827)	(847)
Fixed Maturities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		145,181	144,505
Less than 12 months, Gross Unrealized Losses		(1,051)	(656)
12 months or longer, Fair Value	[1]	2,974	3,216
12 months or longer, Gross Unrealized Losses	[1]	(21)	(53)
Total, Fair Value		148,155	147,721
Total, Gross Unrealized Losses		(1,072)	(709)
Fixed Maturities | Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		42,441	60,518
Less than 12 months, Gross Unrealized Losses		(231)	(558)
Total, Fair Value		42,441	60,518
Total, Gross Unrealized Losses		$ (231)	$ (558)

[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.