Comprehensive Income - Reclassifications Out of Accumulated Other 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
Available for sale ("AFS") securities:
Income before income taxes		$ 63,707		$ 84,736		$ 113,779	$ 165,706
Related income tax effect		(11,021)		(17,529)		(20,910)	(34,857)
Net income		52,686	$ 40,183	67,207	$ 63,642	92,869	130,849
Cash flow hedge:
(gains) included in net income		63,707		84,736		113,779	165,706
Related income tax effect		11,021		17,529		20,910	34,857
Pension plan:
Income before income taxes		63,707		84,736		113,779	165,706
Related income tax effect		(11,021)		(17,529)		(20,910)	(34,857)
Net income		$ 52,686	$ 40,183	$ 67,207	$ 63,642	92,869	$ 130,849
Reclassification out of Accumulated Other Comprehensive Income [Member]
Available for sale ("AFS") securities:
Net income						966
Pension plan:
Net income						966
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains/Losses on AFS Securities [Member]
Available for sale ("AFS") securities:
Net reclassification adjustment for losses (gains) included in net income						(1,641)
Income before income taxes						(1,641)
Related income tax effect						382
Net income						(1,259)
Cash flow hedge:
(gains) included in net income						(1,641)
Related income tax effect						(382)
Pension plan:
Income before income taxes						(1,641)
Related income tax effect						382
Net income						(1,259)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss) Net Cash Flow Hedge Parent [Member]
Available for sale ("AFS") securities:
Income before income taxes						0
Related income tax effect						0
Cash flow hedge:
Net reclassification adjustment for losses						0
(gains) included in net income						0
Related income tax effect						0
Pension plan:
Income before income taxes						0
Related income tax effect						0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Available for sale ("AFS") securities:
Income before income taxes						2,884
Related income tax effect						(659)
Net income						2,225
Cash flow hedge:
(gains) included in net income						2,884
Related income tax effect						659
Pension plan:
Recognized net actuarial loss	[1]					2,884
Income before income taxes						2,884
Related income tax effect						(659)
Net income						$ 2,225

[1] ThisAOCI component is included in the computation of net periodic pension cost (see Note 16, Employee Benefit Plans)