Investment Securities Available for Sale (Schedule of Gross Unrealized Losses on Investment Securities and Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	$ 6,001,368	$ 1,379,662
Less than 12 Months, Gross Unrealized Losses	(109,940)	(16,669)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	6,001,368	1,379,662
Total, Gross Unrealized Losses	(109,940)	(16,669)
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	784,433	566,896
Less than 12 Months, Gross Unrealized Losses	(11,069)	(5,925)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	784,433	566,896
Total, Gross Unrealized Losses	(11,069)	(5,925)
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	4,214,454
Less than 12 Months, Gross Unrealized Losses	(77,253)
12 Months or Longer, Fair Value	0
12 Months or Longer, Gross Unrealized Losses	0
Total, Fair Value	4,214,454
Total, Gross Unrealized Losses	(77,253)
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	770,319	803,429
Less than 12 Months, Gross Unrealized Losses	(10,635)	(10,576)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	770,319	803,429
Total, Gross Unrealized Losses	(10,635)	(10,576)
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	176,469
Less than 12 Months, Gross Unrealized Losses	(7,348)
12 Months or Longer, Fair Value	0
12 Months or Longer, Gross Unrealized Losses	0
Total, Fair Value	176,469
Total, Gross Unrealized Losses	(7,348)
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	9,498	9,337
Less than 12 Months, Gross Unrealized Losses	(6)	(168)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Total, Fair Value	9,498	9,337
Total, Gross Unrealized Losses	(6)	$ (168)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months, Fair Value	46,195
Less than 12 Months, Gross Unrealized Losses	(3,629)
12 Months or Longer, Fair Value	0
12 Months or Longer, Gross Unrealized Losses	0
Total, Fair Value	46,195
Total, Gross Unrealized Losses	$ (3,629)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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