Summary of Cumulative Amounts Related to Credit Loss Portion of Other-Than-Temporary Impairment Losses on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 4,183	$ 2,362	$ 4,183	$ 2,362
Additions for credit loss impairments recognized in the current period on securities not previously impaired		1,821		1,821
Additions for credit loss impairments recognized in the current period on securities previously impaired
Reductions for credit loss impairments previously recognized on securities sold during the period	(1,457)		(1,457)
Ending balance	$ 2,726	$ 4,183	$ 2,726	$ 4,183