Fair Value Measurements, Fair Value Of Financial Instruments as Measured On a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020		Jun. 30, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Assets: [Abstract]
Trading securities	$ 2,245,480			$ 1,623,921
Available for sale securities	12,817,269			11,269,643
Fair value option securities	134,756			1,098,577
Mortgage servicing rights	97,644		$ 97,971	201,886		$ 193,661	$ 208,308	$ 259,254
Derviative contracts, net of cash collateral, Assets, Fair value	593,568			323,375
Liabilities: [Abstract]
Derivative contracts, net of cash collateral, Liabilities, Fair Value	446,328			251,128
U.S. government agency debentures [Member]
Assets: [Abstract]
Trading securities	5,181			44,264
Residential agency mortgage-backed securities [Member]
Assets: [Abstract]
Trading securities	2,150,759			1,504,651
Available for sale securities	9,384,998			8,046,096
Fair value option securities	134,756			1,088,660
Municipal and other tax-exempt securities [Member]
Assets: [Abstract]
Trading securities	30,533			26,196
Available for sale securities	51,601			1,861
Asset-backed securities [Member]
Assets: [Abstract]
Trading securities	0			14,084
U.S. Treasury [Member]
Assets: [Abstract]
Available for sale securities	509			1,600
Fair value option securities	0			9,917
Residential non-agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	34,873			41,609
Commercial agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	3,334,409			3,178,005
Other debt securities [Member]
Assets: [Abstract]
Trading securities	59,007			34,726
Available for sale securities	10,879			472
Fair Value, Recurring [Member]
Assets: [Abstract]
Trading securities	2,245,480			1,623,921
Available for sale securities	12,817,269			11,269,643
Fair value option securities				1,098,577
Residential mortgage loans held for sale	295,290			182,271
Mortgage servicing rights	97,644	[1]		201,886	[2]
Derviative contracts, net of cash collateral, Assets, Fair value	593,568	[3]		323,375	[4]
Liabilities: [Abstract]
Derivative contracts, net of cash collateral, Liabilities, Fair Value	446,328	[3]		251,128	[4]
Fair Value, Recurring [Member] | U.S. government agency debentures [Member]
Assets: [Abstract]
Trading securities	5,181			44,264
Fair Value, Recurring [Member] | Residential agency mortgage-backed securities [Member]
Assets: [Abstract]
Trading securities	2,150,759			1,504,651
Available for sale securities	9,384,998			8,046,096
Fair value option securities	134,756			1,088,660
Fair Value, Recurring [Member] | Municipal and other tax-exempt securities [Member]
Assets: [Abstract]
Trading securities	30,533			26,196
Available for sale securities	51,601			1,861
Fair Value, Recurring [Member] | Asset-backed securities [Member]
Assets: [Abstract]
Trading securities				14,084
Fair Value, Recurring [Member] | U.S. Treasury [Member]
Assets: [Abstract]
Available for sale securities	509			1,600
Fair value option securities				9,917
Fair Value, Recurring [Member] | Residential non-agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	34,873			41,609
Fair Value, Recurring [Member] | Commercial agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	3,334,409			3,178,005
Fair Value, Recurring [Member] | Other debt securities [Member]
Assets: [Abstract]
Trading securities	59,007			34,726
Available for sale securities	10,879			472
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	509			1,600
Fair value option securities				9,917
Residential mortgage loans held for sale	0			0
Mortgage servicing rights	0	[1]		0	[2]
Derviative contracts, net of cash collateral, Assets, Fair value	11,713	[3]		8,944	[4]
Liabilities: [Abstract]
Derivative contracts, net of cash collateral, Liabilities, Fair Value	0	[3]		0	[4]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. government agency debentures [Member]
Assets: [Abstract]
Trading securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Residential agency mortgage-backed securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	0			0
Fair value option securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Municipal and other tax-exempt securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset-backed securities [Member]
Assets: [Abstract]
Trading securities				0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Treasury [Member]
Assets: [Abstract]
Available for sale securities	509			1,600
Fair value option securities				9,917
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Residential non-agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other debt securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets: [Abstract]
Trading securities	2,245,480			1,623,921
Available for sale securities	12,816,288			11,267,571
Fair value option securities				1,088,660
Residential mortgage loans held for sale	286,785			173,958
Mortgage servicing rights	0	[1]		0	[2]
Derviative contracts, net of cash collateral, Assets, Fair value	581,855	[3]		314,431	[4]
Liabilities: [Abstract]
Derivative contracts, net of cash collateral, Liabilities, Fair Value	446,328	[3]		251,128	[4]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. government agency debentures [Member]
Assets: [Abstract]
Trading securities	5,181			44,264
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Residential agency mortgage-backed securities [Member]
Assets: [Abstract]
Trading securities	2,150,759			1,504,651
Available for sale securities	9,384,998			8,046,096
Fair value option securities	134,756			1,088,660
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal and other tax-exempt securities [Member]
Assets: [Abstract]
Trading securities	30,533			26,196
Available for sale securities	51,601			1,861
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset-backed securities [Member]
Assets: [Abstract]
Trading securities				14,084
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Treasury [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair value option securities				0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Residential non-agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	34,873			41,609
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	3,334,409			3,178,005
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other debt securities [Member]
Assets: [Abstract]
Trading securities	59,007			34,726
Available for sale securities	10,407			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	472			472
Fair value option securities				0
Residential mortgage loans held for sale	8,505			8,313
Mortgage servicing rights	97,644	[1]		201,886	[2]
Derviative contracts, net of cash collateral, Assets, Fair value	0	[3]		0	[4]
Liabilities: [Abstract]
Derivative contracts, net of cash collateral, Liabilities, Fair Value	0	[3]		0	[4]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. government agency debentures [Member]
Assets: [Abstract]
Trading securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential agency mortgage-backed securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	0			0
Fair value option securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal and other tax-exempt securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities				0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset-backed securities [Member]
Assets: [Abstract]
Trading securities				0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Treasury [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair value option securities				0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential non-agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial agency mortgage-backed securities [Member]
Assets: [Abstract]
Available for sale securities	0			0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other debt securities [Member]
Assets: [Abstract]
Trading securities	0			0
Available for sale securities	$ 472			$ 472

[1] A reconciliation of the beginning and ending fair value of mortgage servicing rights and disclosures of significant assumptions used to determine fair value are presented in Note 5, Mortgage Banking Activities.

[2] A reconciliation of the beginning and ending fair value of mortgage servicing rights and disclosures of significant assumptions used to determine fair value are presented in Note 5, Mortgage Banking Activities.

[3] See Note 3 for detail of fair value of derivative contracts by contract type. Derivative contracts in asset positions that were valued based on quoted prices in active markets for identical instruments (Level 1) are primarily exchange-traded interest rate derivative contracts. Derivative contracts in liability positions that were valued using quoted prices in active markets for identical instruments are exchange-traded interest rate, energy and agricultural derivative contracts, fully offset by cash margin.

[4] See Note 3 for detail of fair value of derivative contracts by contract type. Derivative contracts based on quoted prices in active markets for identical instruments (Level 1) are exchange-traded interest rate and energy derivative contracts, net of cash margin. Derivative contracts in liability positions that were valued using quoted prices in active markets for identical instruments (Level 1) are exchange-traded interest rate and agricultural contracts, fully offset by cash margin.