Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020		Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	$ 12,817,269		$ 11,269,643
Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	10,879		472
Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	12,817,269		11,269,643
Residential mortgage loans held for sale	295,290		182,271
Fair Value, Recurring [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	10,879		472
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	472		472
Residential mortgage loans held for sale	8,505		8,313
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	$ 472		$ 472
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average Yields On Comparable Short-term Taxable Securities Maximum	2.00%		3.00%
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential mortgage loans held for sale [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liquidity Discount on Mortgage Loans Qualifying for Sale to US Govt Agencies	90.24%		95.23%
Interest Rate Spread [Member] | Discounted Cash Flow [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	$ 472		$ 472
Liquidity discount [Member] | Quoted prices of loans sold in securitization transactions, with a liquidity discount applied [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential mortgage loans held for sale [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Residential mortgage loans held for sale	$ 8,505		$ 8,313
Minimum [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Range of Interest Rates used to Value Securities	5.61%	[1]	7.08%	[2]
Range of fair value as a percentage of par value	94.32%	[3]	94.40%	[4]
Maximum [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Range of Interest Rates used to Value Securities	5.61%	[1]	7.08%	[2]
Range of fair value as a percentage of par value	94.32%	[3]	94.40%	[4]
Weighted Average [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Range of Interest Rates used to Value Securities	5.61%	[1]	7.08%	[2]
Range of fair value as a percentage of par value	94.32%	[3]	94.40%	[4]

[1] Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 2 percent.

[2] Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 3 percent.

[3] Represents fair value as a percentage of par value.

[4] Represents fair value as a percentage of par value.