Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2012
Schedule Of Marketable Securities [Line Items]
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	98.20%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	67.70%
Securities Pledged as Collateral	$ 3,500,000	$ 3,800,000
Available for Sale, Estimated Fair Value	10,536,532	$ 10,489,009
Available for sale securities reclassified to held to maturity, unrealized gain			$ 165,700
Available for sale securities reclassified to held to maturity, unrealized gain net of tax			107,700
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	8,900
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 7,100
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Available for Sale, Estimated Fair Value			$ 2,300,000