Derivative Financial Instruments (Schedule fair value of derivative instruments) (Detail) - USD ($)	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	$ 6,009,067,000		$ 6,198,556,000
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	388,439,000		146,037,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	12,390,000		4,779,000
Asset Derivatives, Fair Value, Less: Cash collateral posted	32,159,000		8,100,000
Gross derivative instruments, before netting (1)	343,890,000		133,158,000
Liability Derivatives, Notional Amount	4,882,080,000		4,722,801,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	18,247,000		13,813,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	12,390,000		4,779,000
Liability Derivatives, Fair Value, Less: Cash collateral posted	5,085,000		1,871,000
Derivative liability	772,000		7,163,000
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	345,502,000		317,486,000
Long-term Debt [Member]
Derivatives, Fair Value [Line Items]
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	45,502,000	$ 48,200,000	17,486,000
CME [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value, Less: Cash collateral posted	82,800,000
Gross derivative instruments, before netting (1)	0
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	44,549,000		13,012,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	44,549,000		12,879,000
Gross derivative instruments, before netting (1)	0		133,000
Derivative Liability, Fair Value, Gross Liability	17,568,000		6,710,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	17,475,000		6,650,000
Derivative liability	93,000		60,000
Designated as Hedging Instrument [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	1,150,000,000		1,225,000,000
Derivative Asset, Fair Value, Gross Asset	44,114,000		11,855,000
Liability Derivatives, Notional Amount	25,000,000		300,000,000
Derivative Liability, Fair Value, Gross Liability	145,000		3,153,000
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	4,859,067,000		4,973,556,000
Asset Derivatives, Fair Value, Positions not subject to master netting agreement	344,325,000		134,182,000
Liability Derivatives, Notional Amount	4,857,080,000		4,422,801,000
Liability Derivatives, Fair Value, Positions not subject to master netting agreement	18,102,000		10,660,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, CME [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	60,100,000		1,100,000,000
Liability Derivatives, Notional Amount	3,400,000,000		2,600,000,000
Not Designated as Hedging Instrument [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	4,642,373,000		4,869,139,000
Derivative Asset, Fair Value, Gross Asset	341,275,000		133,455,000
Liability Derivatives, Notional Amount	4,505,792,000		4,090,522,000
Derivative Liability, Fair Value, Gross Liability	17,212,000		9,732,000
Not Designated as Hedging Instrument [Member] | Credit Risk Contract [Member] | RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	95,400,000		65,700,000
Not Designated as Hedging Instrument [Member] | Credit Risk Contract [Member] | RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount	317,800,000		223,400,000
Not Designated as Hedging Instrument [Member] | Mortgage Banking Derivatives [Member] | RPA-Out [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	105,605,000		27,873,000
Derivative Asset, Fair Value, Gross Asset	2,717,000		329,000
Not Designated as Hedging Instrument [Member] | Mortgage Banking Derivatives [Member] | RPA-In [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Notional Amount	0		57,000,000
Derivative Liability, Fair Value, Gross Liability	0		110,000
Not Designated as Hedging Instrument [Member] | Other Contract [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Notional Amounts	111,089,000		76,544,000
Derivative Asset, Fair Value, Gross Asset	333,000		398,000
Liability Derivatives, Notional Amount	351,288,000		275,279,000
Derivative Liability, Fair Value, Gross Liability	890,000		$ 818,000
Interest Rate Lock Commitments [Member]
Derivatives, Fair Value [Line Items]
Other Commitment	$ 10,800,000