Securities - Net Gains and Losses on Trading Account Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 506	$ 505
Net mark-to-market gains (losses)	4	(36)
Net gain (loss) on trading account securities	$ 510	$ 469

This item represents the difference between the gross amount of gains (losses) included in earnings for transfers of securities from the available-for-sale category into the trading category. Such net gain (loss) included in earnings represents the net unrealized gain (loss) which had been included in other comprehensive income and is associated with the securities transferred from the available-for-sale category to the trading category.

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesTransfersToTradingGainsLossesNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration