Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	$ 12,694,277	$ 11,269,643
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 Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	12,694,277	11,269,643
Residential mortgage loans held for sale	204,720	182,271
Fair Value, Recurring [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]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available for sale securities	472	472
Residential mortgage loans held for sale	9,574	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	3.00%	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	95.24%	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	$ 9,574	$ 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	6.69%	7.08%
Range of fair value as a percentage of par value	94.38%	94.40%
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	6.69%	7.08%
Range of fair value as a percentage of par value	94.38%	94.40%
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	6.69%	7.08%
Range of fair value as a percentage of par value	94.38%	94.40%