Comprehensive Income - Components of Total Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income	$ 52,686	$ 40,183	$ 67,207	$ 63,642	$ 92,869	$ 130,849
AFS securities with OTTI charges during the period	0		(75)		0	(75)
Related income tax effect	0		17		0	17
Less: OTTI charges recognized in net income	0		75		0	75
Related income tax benefit	0		(17)		0	(17)
Reclassification of previous noncredit OTTI to credit OTTI	0		2,188		0	2,188
Related income tax benefit	0		(510)		0	(510)
Net unrealized (losses) gains on AFS securities with OTTI	0		1,678		0	1,678
Change in net unrealized gain on AFS securities arising during the period	37,756		21,141		60,859	43,379
Related income tax effect	(8,797)		(4,925)		(14,180)	(10,107)
Net reclassification adjustment for (gains) losses included in net income	(1,466)		130		(1,641)	(218)
Related income tax expense (benefit)	341		(30)		382	51
Total AFS securities - all other	27,834		16,316		45,420	33,105
Net effect of AFS securities on other comprehensive income	27,834		17,994		45,420	34,783
Cash flow hedge derivatives:
Unrealized loss on cash flow hedge	(1,659)		0		(1,659)	0
Related income tax effect	387		0		387	0
Net effect of cash flow hedge derivatives on other comprehensive income	(1,272)		0		(1,272)	0
Pension plan:
Recognized net actuarial loss	1,442		1,184		2,884	2,355
Related income tax benefit	(329)		(275)		(659)	(536)
Net effect of change in pension plan asset on other comprehensive income	1,113		909		2,225	1,819
Net current-period other comprehensive income, net of tax	27,675	18,698	18,903	17,699	46,373	36,602
Total Comprehensive Income	$ 80,361	$ 58,881	$ 86,110	$ 81,341	$ 139,242	$ 167,451