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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	Jun. 30, 2022	Dec. 31, 2021
Amortized Cost
Within one year, amortized cost	$ 232,605
1 to 5 years, amortized cost	705,828
5 to 10 years, amortized cost	450,876
More than 10 years, amortized cost	9,624,608
Amortized Cost	11,013,917	$ 10,991,506
Fair Value
Within one year, fair value	232,551
1 to 5 years, fair value	673,035
5 to 10 years, fair value	415,387
More than 10 years, fair value	8,568,877
Total, fair value	9,889,850	10,918,329
U.S. Treasury securities
Amortized Cost
Within one year, amortized cost	21,282
1 to 5 years, amortized cost	343,616
5 to 10 years, amortized cost	49,846
More than 10 years, amortized cost	0
Amortized Cost	414,744	120,465
Fair Value
Within one year, fair value	21,282
1 to 5 years, fair value	322,983
5 to 10 years, fair value	42,319
More than 10 years, fair value	0
Total, fair value	386,584	117,838
U.S. Government agency securities
Amortized Cost
Within one year, amortized cost	456
1 to 5 years, amortized cost	22,420
5 to 10 years, amortized cost	29,990
More than 10 years, amortized cost	0
Amortized Cost	52,866	53,214
Fair Value
Within one year, fair value	457
1 to 5 years, fair value	20,581
5 to 10 years, fair value	29,857
More than 10 years, fair value	0
Total, fair value	50,895	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	727
5 to 10 years, amortized cost	5
More than 10 years, amortized cost	712,758
Amortized Cost	713,490	790,329
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	727
5 to 10 years, fair value	5
More than 10 years, fair value	631,239
Total, fair value	631,971	779,633
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	127,499
More than 10 years, amortized cost	8,016,579
Amortized Cost	8,144,078	8,063,890
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	0
5 to 10 years, fair value	124,738
More than 10 years, fair value	7,117,903
Total, fair value	7,242,641	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	114
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	826,465
Amortized Cost	826,579	951,691
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	114
5 to 10 years, fair value	0
More than 10 years, fair value	752,972
Total, fair value	753,086	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	338,951
5 to 10 years, amortized cost	234,679
More than 10 years, amortized cost	68,806
Amortized Cost	642,436	479,420
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	328,630
5 to 10 years, fair value	209,726
More than 10 years, fair value	66,763
Total, fair value	605,119	481,744
Asset-backed securities
Amortized Cost
Within one year, amortized cost	201,366
1 to 5 years, amortized cost	0
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	201,366	514,188
Fair Value
Within one year, fair value	201,366
1 to 5 years, fair value	0
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	201,366	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	8,857
More than 10 years, amortized cost	0
Amortized Cost	18,358	18,309
Fair Value
Within one year, fair value	9,446
1 to 5 years, fair value	0
5 to 10 years, fair value	8,742
More than 10 years, fair value	0
Total, fair value	$ 18,188	$ 18,801

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