Estimated Fair Value, Gross Unrealized Losses, Other-Than-Temporary Impairments and Number of Securities Where Fair Value had Declined Below Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Investment		Dec. 31, 2012 Investment
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	$ 491,881		$ 82,293
Gross Unrealized Losses/ OTTI	(20,627)	[1]	(3,815)	[1]
Number of Securities	175		52
Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	20,292		45,481
Gross Unrealized Losses/ OTTI	(895)	[1]	(3,111)	[1]
Less than or equal to six months
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	460,548		29,007
Gross Unrealized Losses/ OTTI	(18,533)	[1]	(413)	[1]
Decline > 20%
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	5,499		3,655
Gross Unrealized Losses/ OTTI	(1,501)	[1]	(1,180)	[1]
Number of Securities	1		1
Decline > 20% | Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value			3,655
Gross Unrealized Losses/ OTTI			(1,180)	[1]
Number of Securities			1
Decline > 20% | Less than or equal to six months
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	5,499
Gross Unrealized Losses/ OTTI	$ (1,501)	[1]
Number of Securities	1

[1] Subsequent unrealized gains (losses) on OTTI securities are included in OCI-OTTI.