Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (1,165)		$ (1,494)		$ (1,062)	$ (2,282)
Interest income (expense)	(155,230)		(131,963)		(302,278)	(260,535)
Salaries and employee benefits	(21)		(27)		(41)	(54)
Income tax (expense) benefit	14,359		4,345		27,463	9,302
Net income	(62,964)	$ (56,276)	(44,001)	$ (47,983)	(119,240)	(91,984)
Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	4		(8)		294	(588)
Unrealized Gains and Losses on Available- for-Sale Debt Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	892		1,136		813	1,730
Accumulated Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(435)		(402)		(786)	(858)
Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(16)		(21)		(31)	(41)
Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	445		705		290	243
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income (expense)	6		(10)		391	(779)
Income tax (expense) benefit	(2)		2		(97)	191
Net income	4		(8)		294	(588)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Unrealized Gains and Losses on Available- for-Sale Debt Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	1,165		1,494		1,062	2,282
Income tax (expense) benefit	(273)		(358)		(249)	(552)
Net income	892		1,136		813	1,730
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income (expense)	(564)		(521)		(1,021)	(1,112)
Income tax (expense) benefit	129		119		235	254
Net income	(435)		(402)		(786)	(858)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	(21)		(27)		(41)	(54)
Income tax (expense) benefit	5		6		10	13
Net income	$ (16)		$ (21)		$ (31)	$ (41)