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	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 5,154	$ 3,332	$ 3,321
Additions for credit loss impairments recognized in the current period on securities not previously impaired		1,822
Additions for credit loss impairments recognized in the current period on securities previously impaired			11
Reductions for credit loss impairments previously recognized on securities sold during the period	(2,793)
Ending balance	$ 2,361	$ 5,154	$ 3,332