Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (424)			$ (135)			$ (1,486)	$ (2,417)
Interest income (expense)	(153,096)			(130,842)			(455,374)	(391,377)
Salaries and employee benefits	(20)			(27)			(61)	(81)
Income tax (expense) benefit	13,254			5,325			40,717	14,627
Net income	(69,781)	$ (62,964)	$ (56,276)	(51,348)	$ (44,001)	$ (47,983)	(189,021)	(143,332)
Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	13			171			307	(417)
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	335			117			1,147	1,847
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	(992)			(482)			(1,778)	(1,340)
Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(15)			(20)			(46)	(61)
Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(659)			(214)			(370)	29
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)	21			226			412	(553)
Income tax (expense) benefit	(8)			(55)			(105)	136
Net income	13			171			307	(417)
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	424			135			1,486	2,417
Income tax (expense) benefit	(89)			(18)			(339)	(570)
Net income	335			117			1,147	1,847
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)	(1,384)			(628)			(2,405)	(1,740)
Income tax (expense) benefit	392			146			627	400
Net income	(992)			(482)			(1,778)	(1,340)
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	(20)			(27)			(61)	(81)
Income tax (expense) benefit	5			7			15	20
Net income	$ (15)			$ (20)			$ (46)	$ (61)