Derivative Financial Instruments (Schedule fair value of derivative instruments) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		$ 6,198,556	$ 5,683,199
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		146,037	332,856
Derivative Asset, Fair Value, Amount Offset Against Collateral		4,779	7,522
Asset Derivatives, Fair Value, Less: Cash collateral posted		8,100	33,043
Derivative asset		133,158	292,291
Liability Derivatives, Notional Amount		4,722,801	4,741,836
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		13,813	12,354
Derivative Liability, Fair Value, Amount Offset Against Collateral		4,779	7,522
Liability Derivatives, Fair Value, Less: Cash collateral posted		1,871	4,485
Derivative liability		7,163	347
CME SWAPS MARKETS (CME) [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value, Less: Cash collateral posted	$ 79,000
Cross Currency Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	80,500	65,700
Liability Derivatives, Notional Amount	338,900	223,400
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	40,565	13,012
Derivative Asset, Fair Value, Amount Offset Against Collateral	40,565	12,879
Derivative asset	0	133
Derivative Liability, Fair Value, Gross Liability	12,007	6,710
Derivative Liability, Fair Value, Amount Offset Against Collateral	12,007	6,650
Derivative liability	0	60
Designated as Hedging Instrument [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		1,225,000	1,000,000
Derivative Asset, Fair Value, Gross Asset		11,855	39,641
Liability Derivatives, Notional Amount		300,000	25,000
Derivative Liability, Fair Value, Gross Liability		3,153	103
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,973,556	4,683,199
Asset Derivatives, Fair Value, Positions not subject to master netting agreement		134,182	293,215
Liability Derivatives, Notional Amount		4,422,801	4,716,836
Liability Derivatives, Fair Value, Positions not subject to master netting agreement		10,660	12,251
Not Designated as Hedging Instrument [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		4,869,139	4,533,441
Derivative Asset, Fair Value, Gross Asset		133,455	292,096
Liability Derivatives, Notional Amount		4,090,522	4,356,339
Derivative Liability, Fair Value, Gross Liability		9,732	11,874
Not Designated as Hedging Instrument [Member] | Mortgage Banking Derivatives [Member] | RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		27,873	40,771
Derivative Asset, Fair Value, Gross Asset		329	855
Not Designated as Hedging Instrument [Member] | Mortgage Banking Derivatives [Member] | RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount		57,000	0
Derivative Liability, Fair Value, Gross Liability		110	0
Not Designated as Hedging Instrument [Member] | Other Contract [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts		76,544	108,987
Derivative Asset, Fair Value, Gross Asset		398	264
Liability Derivatives, Notional Amount		275,279	360,497
Derivative Liability, Fair Value, Gross Liability		818	$ 377
Not Designated as Hedging Instrument [Member] | CME SWAPS MARKETS (CME) [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	100,000	1,100,000
Liability Derivatives, Notional Amount	3,200,000	$ 2,600,000
Interest Rate Lock Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment	$ 2,000