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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (135)	$ (2,972)	$ (2,417)	$ (7,547)
Interest income (expense)	(130,842)	(108,478)	(391,377)	(318,612)
Salaries and employee benefits	(27)	(27)	(81)	(81)
Income tax (expense) benefit	5,325	11,459	14,627	32,534
Net income	(51,348)	(39,372)	(143,332)	(114,218)
Total reclassifications for the period	(214)	680	29	767
Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	171	(887)	(417)	(3,077)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	117	1,883	1,847	4,787
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	(482)	(300)	(1,340)	(893)
Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(20)	(16)	(61)	(50)
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)	226	(1,429)	(553)	(4,962)
Income tax (expense) benefit	(55)	542	136	1,885
Net income	171	(887)	(417)	(3,077)
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	135	2,972	2,417	7,547
Income tax (expense) benefit	(18)	(1,089)	(570)	(2,760)
Net income	117	1,883	1,847	4,787
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)	(628)	(456)	(1,740)	(1,358)
Income tax (expense) benefit	146	156	400	465
Net income	(482)	(300)	(1,340)	(893)
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)	(81)	(81)
Income tax (expense) benefit	7	11	20	31
Net income	$ (20)	$ (16)	$ (61)	$ (50)