Investment Securities Available for Sale (Summary of Available for Sale Investment Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 9,398,505	$ 7,801,864
Gross Unrealized Gains	103,007	177,243
Gross Unrealized Losses	(59,342)	(16,669)
Fair Value	9,442,170	7,962,438
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	120,119	20,257
Gross Unrealized Gains	355	0
Gross Unrealized Losses	(2,243)	0
Fair Value	118,231	20,257
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,161	79,638
Gross Unrealized Gains	1,954	2,682
Gross Unrealized Losses	0	0
Fair Value	56,115	82,320
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	981,107	1,216,012
Gross Unrealized Gains	2,084	7,930
Gross Unrealized Losses	(7,077)	(5,925)
Fair Value	976,114	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,670,497	4,865,858
Gross Unrealized Gains	75,773	134,188
Gross Unrealized Losses	(37,855)	0
Fair Value	6,708,415	5,000,046
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,103,547	1,245,644
Gross Unrealized Gains	9,644	15,309
Gross Unrealized Losses	(9,145)	(10,576)
Fair Value	1,104,046	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	450,814	354,244
Gross Unrealized Gains	12,750	16,677
Gross Unrealized Losses	(3,022)	0
Fair Value	460,542	370,921
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,260	20,211
Gross Unrealized Gains	447	457
Gross Unrealized Losses	0	(168)
Fair Value	$ 18,707	$ 20,500

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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