Securities (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	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	67.80%
Securities pledged as collateral	$ 3,800,000	$ 3,900,000
Securities available for sale, at estimated fair value	10,489,009	10,203,277
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	11,600	27,700
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	7,500	$ 18,000
Other than temporary impairment, credit losses recognized in earnings, credit losses on debt securities held	$ 0
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Securities available for sale, at estimated fair value			$ 2,300,000