Derivative Financial Instruments (Schedule fair value of derivative instruments) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		$ 6,198,556	$ 4,859,561
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		146,037	46,278
Derivative Asset, Fair Value, Amount Offset Against Collateral		4,779	2,495
Asset Derivatives, Fair Value, Less: Cash collateral posted		8,100	4,936
Derivative asset		133,158	38,847
Liability Derivatives, Notional Amount		4,722,801	3,801,586
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		13,813	39,010
Derivative Liability, Fair Value, Amount Offset Against Collateral		4,779	2,495
Liability Derivatives, Fair Value, Less: Cash collateral posted		1,871	0
Derivative liability		7,163	36,515
Interest Rate Floor [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 1,000,000
CME SWAPS MARKETS (CME) [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value, Less: Cash collateral posted	56,600
Cross Currency Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	65,700		65,000
Liability Derivatives, Notional Amount	223,400		96,300
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13,012		9,928
Derivative Asset, Fair Value, Amount Offset Against Collateral	12,879		7,431
Derivative asset	133		2,497
Derivative Liability, Fair Value, Gross Liability	6,710		2,566
Derivative Liability, Fair Value, Amount Offset Against Collateral	6,650		2,495
Derivative liability	60		71
Designated as Hedging Instrument [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		1,225,000	325,000
Derivative Asset, Fair Value, Gross Asset		11,855	3,050
Liability Derivatives, Notional Amount		300,000	0
Derivative Liability, Fair Value, Gross Liability		3,153	0
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,973,556	4,534,561
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		134,182	43,228
Liability Derivatives, Notional Amount		4,422,801	3,801,586
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		10,660	39,010
Not Designated as Hedging Instrument [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,869,139	4,435,530
Derivative Asset, Fair Value, Gross Asset		133,455	42,205
Liability Derivatives, Notional Amount		4,090,522	3,643,985
Derivative Liability, Fair Value, Gross Liability		9,732	38,029
Not Designated as Hedging Instrument [Member] \| Mortgage Banking Derivatives [Member] \| RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		27,873	13,599
Derivative Asset, Fair Value, Gross Asset		329	226
Not Designated as Hedging Instrument [Member] \| Mortgage Banking Derivatives [Member] \| RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount		57,000	17,000
Derivative Liability, Fair Value, Gross Liability		110	293
Not Designated as Hedging Instrument [Member] \| Other Contract [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		76,544	85,432
Derivative Asset, Fair Value, Gross Asset		398	797
Liability Derivatives, Notional Amount		275,279	140,601
Derivative Liability, Fair Value, Gross Liability		$ 818	688
Not Designated as Hedging Instrument [Member] \| CME SWAPS MARKETS (CME) [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	1,100,000		1,900,000
Liability Derivatives, Notional Amount	2,600,000		$ 1,100,000
Loan Origination Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment	57,000
Interest Rate Lock Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment	$ 7,300

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