Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($) security bond	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security bond
Marketable Securities [Line Items]
Reduction in combined securities portfolio			$ (500,153)
Reduction in combined securities portfolio (as a percent)			(43.40%)
Transfer from held to maturity securities to available for sale securities amortized cost		$ 117,409
Transfer from held to maturity securities to available for sale securities fair value		$ 121,964
Number of bonds transferred to AFS | bond	16
Carrying value of pledged securities	$ 294,585		$ 294,585	$ 939,440
Number of available for sale securities in an unrealized loss position | security	56		56	163
Number of HTM securities in an unrecognized loss position | security	0		0	33
ASU 2017-12
Marketable Securities [Line Items]
Number of bonds transferred from HTM intention to AFS intention under one-time exemption | bond				40