Comprehensive Income - Components of Total Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income	$ 40,183	$ 63,642
Change in net unrealized gain on AFS securities arising during the period	23,103	22,238
Related income tax effect	(5,383)	(5,182)
Net reclassification adjustment for losses (gains) included in net income	(175)	(348)
Related income tax (benefit) expense	41	81
Total AFS securities - all other	17,586	16,789
Net effect of AFS securities on other comprehensive income	17,586	16,789
Pension plan:
Recognized net actuarial loss	1,442	1,171
Related income tax benefit	(330)	(261)
Net effect of change in pension plan asset on other comprehensive income	1,112	910
Net current-period other comprehensive income, net of tax	18,698	17,699
Total Comprehensive Income	$ 58,881	$ 81,341