Securities - Summary of Cumulative Credit-Related OTTI Charges (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 27	$ 17,182
Loss where impairment was not previously recognized	0	0
Additional loss where impairment was previously recognized	0	0
Reduction due to credit impaired securities sold		(17,155)
Ending balance	27	27
Collateralized Debt Obligations [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance		17,155
Loss where impairment was not previously recognized	0	0
Additional loss where impairment was previously recognized	0	0
Reduction due to credit impaired securities sold		(17,155)
Equity Securities [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	27	27
Loss where impairment was not previously recognized	0	0
Additional loss where impairment was previously recognized	0	0
Ending balance	$ 27	$ 27