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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	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Within one year, amortized cost	$ 671,165
1 to 5 years, amortized cost	120,002
5 to 10 years, amortized cost	463,889
More than 10 years, amortized cost	9,238,784
Amortized Cost	10,493,840	$ 7,801,864
Fair Value
Within one year, fair value	671,179
1 to 5 years, fair value	123,758
5 to 10 years, fair value	466,406
More than 10 years, fair value	9,219,728
Total, fair value	10,481,071	7,962,438
U.S. Treasury securities
Amortized Cost
Within one year, amortized cost	20,350
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	99,862
More than 10 years, amortized cost	0
Amortized Cost	120,212	20,257
Fair Value
Within one year, fair value	20,350
1 to 5 years, fair value	0
5 to 10 years, fair value	97,543
More than 10 years, fair value	0
Total, fair value	117,893	20,257
U.S. Government agency securities
Amortized Cost
Within one year, amortized cost	756
1 to 5 years, amortized cost	321
5 to 10 years, amortized cost	52,136
More than 10 years, amortized cost	0
Amortized Cost	53,213	79,638
Fair Value
Within one year, fair value	769
1 to 5 years, fair value	326
5 to 10 years, fair value	53,894
More than 10 years, fair value	0
Total, fair value	54,989	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	952
5 to 10 years, amortized cost	140
More than 10 years, amortized cost	878,096
Amortized Cost	879,188	1,216,012
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	989
5 to 10 years, fair value	146
More than 10 years, fair value	871,082
Total, fair value	872,217	1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within one year, amortized cost	59
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	71,559
More than 10 years, amortized cost	7,208,953
Amortized Cost	7,280,571	4,865,858
Fair Value
Within one year, fair value	60
1 to 5 years, fair value	0
5 to 10 years, fair value	74,540
More than 10 years, fair value	7,196,333
Total, fair value	7,270,933	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	165
More than 10 years, amortized cost	1,060,638
Amortized Cost	1,060,803	1,245,644
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	0
5 to 10 years, fair value	172
More than 10 years, fair value	1,057,316
Total, fair value	1,057,488	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	109,227
5 to 10 years, amortized cost	231,245
More than 10 years, amortized cost	91,097
Amortized Cost	431,569	354,244
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	112,628
5 to 10 years, fair value	230,898
More than 10 years, fair value	94,997
Total, fair value	438,523	370,921
Asset-backed securities
Amortized Cost
Within one year, amortized cost	650,000
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	650,000
Fair Value
Within one year, fair value	650,000
1 to 5 years, fair value	0
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	650,000
Corporate debt securities and other debt securities
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	9,502
5 to 10 years, amortized cost	8,782
More than 10 years, amortized cost	0
Amortized Cost	18,284	20,211
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	9,815
5 to 10 years, fair value	9,213
More than 10 years, fair value	0
Total, fair value	$ 19,028	$ 20,500

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