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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (1,494)	$ (3,075)	$ (2,282)	$ (4,575)
Interest income (expense)	(131,963)	(104,333)	(260,535)	(210,134)
Salaries and employee benefits	(27)	(27)	(54)	(54)
Income tax (expense) benefit	4,345	10,584	9,302	21,075
Net income	(44,001)	(38,854)	(91,984)	(74,846)
Total reclassifications for Net income the period	705	567	243	87
Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for Net income the period	(8)	(1,075)	(588)	(2,190)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for Net income the period	1,136	1,957	1,730	2,904
Accumulated Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for Net income the period	(402)	(298)	(858)	(593)
Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for Net income the period	(21)	(17)	(41)	(34)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	2	659	191	1,343
Net income	(8)	(1,075)	(588)	(2,190)
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)	(10)	(1,734)	(779)	(3,533)
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	1,494	3,075	2,282	4,575
Income tax (expense) benefit	(358)	(1,118)	(552)	(1,671)
Net income	1,136	1,957	1,730	2,904
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)	(521)	(453)	(1,112)	(902)
Income tax (expense) benefit	119	155	254	309
Net income	(402)	(298)	(858)	(593)
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	(27)	(27)	(54)	(54)
Income tax (expense) benefit	6	10	13	20
Net income	$ (21)	$ (17)	$ (41)	$ (34)