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Investment Securities Available for Sale (Amortized Cost and Estimated Fair Value by Contractual Maturity of Investment Securities Available for Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Within One year, Amortized Cost	$ 24,893
1 to 5 Years, Amortized Cost	121,905
5 to 10 Years, Amortized Cost	441,089
More Than 10 years, Amortized Cost	8,240,547
Amortized Cost	8,828,434	$ 7,801,864
Fair Value
Within One Year, Fair Value	24,902
1 to 5 Years, Fair Value	126,273
5 to 10 Years, Fair Value	439,512
More Than 10 years, Fair Value	8,235,070
Total, Fair Value	8,825,757	7,962,438
U.S. Treasury securities
Amortized Cost
Within One year, Amortized Cost	20,261
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	49,824
More Than 10 years, Amortized Cost	0
Amortized Cost	70,085	20,257
Fair Value
Within One Year, Fair Value	20,261
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	46,195
More Than 10 years, Fair Value	0
Total, Fair Value	66,456
U.S. Government agency securities
Amortized Cost
Within One year, Amortized Cost	430
1 to 5 Years, Amortized Cost	1,594
5 to 10 Years, Amortized Cost	77,124
More Than 10 years, Amortized Cost	0
Amortized Cost	79,148	79,638
Fair Value
Within One Year, Fair Value	438
1 to 5 Years, Fair Value	1,623
5 to 10 Years, Fair Value	79,113
More Than 10 years, Fair Value	0
Total, Fair Value	81,174	82,320
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	1,232
5 to 10 Years, Amortized Cost	162
More Than 10 years, Amortized Cost	963,315
Amortized Cost	964,709	1,216,012
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	1,281
5 to 10 Years, Fair Value	169
More Than 10 years, Fair Value	953,732
Total, Fair Value	955,182	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	76
1 to 5 Years, Amortized Cost	87
5 to 10 Years, Amortized Cost	77,685
More Than 10 years, Amortized Cost	6,022,673
Amortized Cost	6,100,521	4,865,858
Fair Value
Within One Year, Fair Value	77
1 to 5 Years, Fair Value	88
5 to 10 Years, Fair Value	80,469
More Than 10 years, Fair Value	6,021,141
Total, Fair Value	6,101,775	5,000,046
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	206
More Than 10 years, Amortized Cost	1,154,974
Amortized Cost	1,155,180	1,245,644
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	215
More Than 10 years, Fair Value	1,157,008
Total, Fair Value	1,157,223	1,250,377
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	4,126
1 to 5 Years, Amortized Cost	109,488
5 to 10 Years, Amortized Cost	227,357
More Than 10 years, Amortized Cost	99,585
Amortized Cost	440,556	354,244
Fair Value
Within One Year, Fair Value	4,126
1 to 5 Years, Fair Value	113,783
5 to 10 Years, Fair Value	224,116
More Than 10 years, Fair Value	103,189
Total, Fair Value	445,214	370,921
Corporate debt securities and other debt securities
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	9,504
5 to 10 Years, Amortized Cost	8,731
More Than 10 years, Amortized Cost	0
Amortized Cost	18,235	20,211
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	9,498
5 to 10 Years, Fair Value	9,235
More Than 10 years, Fair Value	0
Total, Fair Value	$ 18,733	$ 20,500

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