Summary of Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		$ 601,410	$ 827,179
Less than 12 months, Gross Unrealized Losses		(8,533)	(14,772)
12 months or longer, Fair Value	[1]	103,870	41,401
12 months or longer, Gross Unrealized Losses	[1]	(891)	(515)
Total, Fair Value		705,280	868,580
Total, Gross Unrealized Losses		(9,424)	(15,287)
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(379)
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(583)
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(1,747)
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(2,848)
Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Total, Gross Unrealized Losses		(673)
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		57,439	36,798
Less than 12 months, Gross Unrealized Losses		(2,403)	(5,960)
Total, Fair Value		57,439	36,798
Total, Gross Unrealized Losses		(2,403)	(5,960)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		543,971	790,381
Less than 12 months, Gross Unrealized Losses		(6,130)	(8,812)
12 months or longer, Fair Value	[1]	103,870	41,401
12 months or longer, Gross Unrealized Losses	[1]	(891)	(515)
Total, Fair Value		647,841	831,782
Total, Gross Unrealized Losses		(7,021)	(9,327)
Fixed Maturities | U.S. Treasury and Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		39,570	79,496
Less than 12 months, Gross Unrealized Losses		(233)	(321)
Total, Fair Value		39,570	79,496
Total, Gross Unrealized Losses		(233)	(321)
Fixed Maturities | Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		46,861	49,708
Less than 12 months, Gross Unrealized Losses		(369)	(373)
12 months or longer, Fair Value	[1]	670	7,732
12 months or longer, Gross Unrealized Losses	[1]	(10)	(84)
Total, Fair Value		47,531	57,440
Total, Gross Unrealized Losses		(379)	(457)
Fixed Maturities | Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		52,780	63,759
Less than 12 months, Gross Unrealized Losses		(541)	(743)
12 months or longer, Fair Value	[1]	298
12 months or longer, Gross Unrealized Losses	[1]	(17)
Total, Fair Value		53,078	63,759
Total, Gross Unrealized Losses		(558)	(743)
Fixed Maturities | Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		62,737	203,381
Less than 12 months, Gross Unrealized Losses		(493)	(1,404)
12 months or longer, Fair Value	[1]	23,937	4,843
12 months or longer, Gross Unrealized Losses	[1]	(90)	(17)
Total, Fair Value		86,674	208,224
Total, Gross Unrealized Losses		(583)	(1,421)
Fixed Maturities | Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		94,366	118,813
Less than 12 months, Gross Unrealized Losses		(1,090)	(2,005)
12 months or longer, Fair Value	[1]	69,747	21,577
12 months or longer, Gross Unrealized Losses	[1]	(657)	(347)
Total, Fair Value		164,113	140,390
Total, Gross Unrealized Losses		(1,747)	(2,352)
Fixed Maturities | Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		171,621	211,364
Less than 12 months, Gross Unrealized Losses		(2,731)	(3,269)
12 months or longer, Fair Value	[1]	9,218	2,120
12 months or longer, Gross Unrealized Losses	[1]	(117)	(25)
Total, Fair Value		180,839	213,484
Total, Gross Unrealized Losses		(2,848)	(3,294)
Fixed Maturities | Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		76,036	63,860
Less than 12 months, Gross Unrealized Losses		(673)	(697)
12 months or longer, Fair Value	[1]		5,129
12 months or longer, Gross Unrealized Losses	[1]		(42)
Total, Fair Value		76,036	68,989
Total, Gross Unrealized Losses		$ (673)	$ (739)

[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.