Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains									$ (2,060)	$ (9,135)	$ (5,848)
Interest income (expense)	$ (146,225)	$ (130,842)	$ (131,963)	$ (128,572)	$ (118,556)	$ (108,478)	$ (104,333)	$ (105,801)	(537,602)	(437,168)	(402,703)
Salaries and employee benefits									(191)	(159)	(11,203)
Income tax (expense) benefit	3,223	5,325	4,345	4,957	40,405	11,459	10,584	10,491	17,850	72,939	66,162
Net income	$ (47,498)	$ (51,348)	$ (44,001)	$ (47,983)	$ 18,493	$ (39,372)	$ (38,854)	$ (35,992)	(190,830)	(95,725)	(134,264)
Total reclassifications for the period									(273)	1,090	(8,378)
Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period									(113)	(3,803)	(4,001)
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									1,662	6,160	3,693
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									(1,678)	(1,203)	(1,170)
Defined Benefit Pension Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period									(144)	(64)	(6,900)
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)									(150)	(6,135)	(6,453)
Income tax (expense) benefit									37	2,332	2,452
Net income									(113)	(3,803)	(4,001)
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									2,060	9,135	5,848
Income tax (expense) benefit									(398)	(2,975)	(2,155)
Net income									1,662	6,160	3,693
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)									(2,181)	(1,830)	(1,776)
Income tax (expense) benefit									503	627	606
Net income									(1,678)	(1,203)	(1,170)
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									(191)	(159)	(11,203)
Income tax (expense) benefit									47	95	4,303
Net income									$ (144)	$ (64)	$ (6,900)