Securities (Net Gains and Losses on Trading Account Securities) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 1,236	$ 1,109	$ 829
Net mark-to-market gains (losses)	(157)	(53)	0
Net gain on trading account securities	$ 1,079	$ 1,056	$ 829