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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, 2022	Dec. 31, 2021
Amortized Cost
Within one year, amortized cost	$ 368,357
1 to 5 years, amortized cost	596,035
5 to 10 years, amortized cost	473,185
More than 10 years, amortized cost	9,720,183
Amortized Cost	11,157,760	$ 10,991,506
Fair Value
Within one year, fair value	368,369
1 to 5 years, fair value	575,153
5 to 10 years, fair value	448,786
More than 10 years, fair value	9,070,793
Total, fair value	10,463,101	10,918,329
U.S. Treasury securities
Amortized Cost
Within one year, amortized cost	21,353
1 to 5 years, amortized cost	343,202
5 to 10 years, amortized cost	49,842
More than 10 years, amortized cost	0
Amortized Cost	414,397	120,465
Fair Value
Within one year, fair value	21,353
1 to 5 years, fair value	328,920
5 to 10 years, fair value	44,313
More than 10 years, fair value	0
Total, fair value	394,586	117,838
U.S. Government agency securities
Amortized Cost
Within one year, amortized cost	455
1 to 5 years, amortized cost	273
5 to 10 years, amortized cost	52,137
More than 10 years, amortized cost	0
Amortized Cost	52,865	53,214
Fair Value
Within one year, fair value	462
1 to 5 years, fair value	275
5 to 10 years, fair value	51,068
More than 10 years, fair value	0
Total, fair value	51,805	54,201
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	835
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	732,454
Amortized Cost	733,289	790,329
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	845
5 to 10 years, fair value	0
More than 10 years, fair value	685,899
Total, fair value	686,744	779,633
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within one year, amortized cost	11
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	136,502
More than 10 years, amortized cost	8,033,096
Amortized Cost	8,169,609	8,063,890
Fair Value
Within one year, fair value	11
1 to 5 years, fair value	0
5 to 10 years, fair value	134,665
More than 10 years, fair value	7,485,955
Total, fair value	7,620,631	8,012,301
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	129
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	877,233
Amortized Cost	877,362	951,691
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	130
5 to 10 years, fair value	0
More than 10 years, fair value	822,748
Total, fair value	822,878	939,623
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	251,596
5 to 10 years, amortized cost	225,872
More than 10 years, amortized cost	77,400
Amortized Cost	554,868	479,420
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	244,983
5 to 10 years, fair value	209,622
More than 10 years, fair value	76,191
Total, fair value	530,796	481,744
Asset-backed securities
Amortized Cost
Within one year, amortized cost	337,037
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	337,037	514,188
Fair Value
Within one year, fair value	337,037
1 to 5 years, fair value	0
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	337,037	514,188
Corporate debt securities and other debt securities
Amortized Cost
Within one year, amortized cost	9,501
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	8,832
More than 10 years, amortized cost	0
Amortized Cost	18,333	18,309
Fair Value
Within one year, fair value	9,506
1 to 5 years, fair value	0
5 to 10 years, fair value	9,118
More than 10 years, fair value	0
Total, fair value	$ 18,624	$ 18,801

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