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Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 8,828,434	$ 7,801,864
Gross Unrealized Gains	107,263	177,243
Gross Unrealized Losses	(109,940)	(16,669)
Fair Value	8,825,757	7,962,438
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	70,085	20,257
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,629)	0
Fair Value	66,456
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	79,148	79,638
Gross Unrealized Gains	2,026	2,682
Gross Unrealized Losses	0	0
Fair Value	81,174	82,320
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	964,709	1,216,012
Gross Unrealized Gains	1,542	7,930
Gross Unrealized Losses	(11,069)	(5,925)
Fair Value	955,182	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,100,521	4,865,858
Gross Unrealized Gains	78,507	134,188
Gross Unrealized Losses	(77,253)	0
Fair Value	6,101,775	5,000,046
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,155,180	1,245,644
Gross Unrealized Gains	12,678	15,309
Gross Unrealized Losses	(10,635)	(10,576)
Fair Value	1,157,223	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	440,556	354,244
Gross Unrealized Gains	12,006	16,677
Gross Unrealized Losses	(7,348)	0
Fair Value	445,214	370,921
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,235	20,211
Gross Unrealized Gains	504	457
Gross Unrealized Losses	(6)	(168)
Fair Value	$ 18,733	$ 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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