Investment Securities Available for Sale (Amortized Cost and Estimated Fair Value by Contractual Maturity of Investment Securities Available for Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Within One year, Amortized Cost	$ 20,795
1 to 5 Years, Amortized Cost	121,452
5 to 10 Years, Amortized Cost	467,453
More Than 10 years, Amortized Cost	8,788,805
Amortized Cost	9,398,505	$ 7,801,864
Fair Value
Within One Year, Fair Value	20,798
1 to 5 Years, Fair Value	125,798
5 to 10 Years, Fair Value	471,810
More Than 10 years, Fair Value	8,823,764
Total, Fair Value	9,442,170	7,962,438
U.S. Treasury securities
Amortized Cost
Within One year, Amortized Cost	20,260
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	99,859
More Than 10 years, Amortized Cost	0
Amortized Cost	120,119	20,257
Fair Value
Within One Year, Fair Value	20,260
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	97,971
More Than 10 years, Fair Value	0
Total, Fair Value	118,231	20,257
U.S. Government agency securities
Amortized Cost
Within One year, Amortized Cost	430
1 to 5 Years, Amortized Cost	1,595
5 to 10 Years, Amortized Cost	52,136
More Than 10 years, Amortized Cost	0
Amortized Cost	54,161	79,638
Fair Value
Within One Year, Fair Value	432
1 to 5 Years, Fair Value	1,609
5 to 10 Years, Fair Value	54,074
More Than 10 years, Fair Value	0
Total, Fair Value	56,115	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	1,081
5 to 10 Years, Amortized Cost	151
More Than 10 years, Amortized Cost	979,875
Amortized Cost	981,107	1,216,012
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	1,124
5 to 10 Years, Fair Value	158
More Than 10 years, Fair Value	974,832
Total, Fair Value	976,114	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	105
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	61,715
More Than 10 years, Amortized Cost	6,608,677
Amortized Cost	6,670,497	4,865,858
Fair Value
Within One Year, Fair Value	106
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	64,294
More Than 10 years, Fair Value	6,644,015
Total, Fair Value	6,708,415	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	185
More Than 10 years, Amortized Cost	1,103,362
Amortized Cost	1,103,547	1,245,644
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	192
More Than 10 years, Fair Value	1,103,854
Total, Fair Value	1,104,046	1,250,377
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	109,272
5 to 10 Years, Amortized Cost	244,651
More Than 10 years, Amortized Cost	96,891
Amortized Cost	450,814	354,244
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	113,193
5 to 10 Years, Fair Value	246,286
More Than 10 years, Fair Value	101,063
Total, Fair Value	460,542	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	8,756
More Than 10 years, Amortized Cost	0
Amortized Cost	18,260	20,211
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	9,872
5 to 10 Years, Fair Value	8,835
More Than 10 years, Fair Value	0
Total, Fair Value	$ 18,707	$ 20,500

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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