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Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 10,493,840	$ 7,801,864
Gross Unrealized Gains	88,776	177,243
Gross Unrealized Losses	(101,545)	(16,669)
Fair Value	10,481,071	7,962,438
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	120,212	20,257
Gross Unrealized Gains	116	0
Gross Unrealized Losses	(2,435)	0
Fair Value	117,893	20,257
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,213	79,638
Gross Unrealized Gains	1,776	2,682
Gross Unrealized Losses	0	0
Fair Value	54,989	82,320
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	879,188	1,216,012
Gross Unrealized Gains	1,441	7,930
Gross Unrealized Losses	(8,412)	(5,925)
Fair Value	872,217	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,280,571	4,865,858
Gross Unrealized Gains	65,605	134,188
Gross Unrealized Losses	(75,243)	0
Fair Value	7,270,933	5,000,046
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,060,803	1,245,644
Gross Unrealized Gains	7,736	15,309
Gross Unrealized Losses	(11,051)	(10,576)
Fair Value	1,057,488	1,250,377
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	431,569	354,244
Gross Unrealized Gains	11,358	16,677
Gross Unrealized Losses	(4,404)	0
Fair Value	438,523	370,921
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	650,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	650,000
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,284	20,211
Gross Unrealized Gains	744	457
Gross Unrealized Losses	0	(168)
Fair Value	$ 19,028	$ 20,500

X

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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