Investment Securities Available for Sale (Schedule of Amortized Cost and Estimated Fair Value by Contractual Maturity of Investment Securities Available for Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Within one year, amortized cost	$ 55,106
1 to 5 years, amortized cost	1,286,444
5 to 10 years, amortized cost	346,639
More than 10 years, amortized cost	9,486,566
Amortized Cost	11,174,755	$ 11,273,327
Fair Value
Within one year, fair value	54,363
1 to 5 years, fair value	1,201,659
5 to 10 years, fair value	291,498
More than 10 years, fair value	7,689,671
Total, fair value	9,237,191	9,678,103
U.S. Treasury securities
Amortized Cost
Within one year, amortized cost	25,352
1 to 5 years, amortized cost	481,570
5 to 10 years, amortized cost	49,869
More than 10 years, amortized cost	0
Amortized Cost	556,791	515,953
Fair Value
Within one year, fair value	25,352
1 to 5 years, fair value	445,205
5 to 10 years, fair value	38,903
More than 10 years, fair value	0
Total, fair value	509,460	471,813
U.S. Government agency securities
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	52,140
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	52,140	52,411
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	47,506
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	47,506	48,798
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within one year, amortized cost	36
1 to 5 years, amortized cost	267
5 to 10 years, amortized cost	4
More than 10 years, amortized cost	1,040,518
Amortized Cost	1,040,825	904,593
Fair Value
Within one year, fair value	34
1 to 5 years, fair value	257
5 to 10 years, fair value	3
More than 10 years, fair value	891,960
Total, fair value	892,254	792,749
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	18,277
5 to 10 years, amortized cost	78,557
More than 10 years, amortized cost	7,684,917
Amortized Cost	7,781,751	8,144,374
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	17,284
5 to 10 years, fair value	70,862
More than 10 years, fair value	6,173,606
Total, fair value	6,261,752	6,895,070
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	58
5 to 10 years, amortized cost	10,944
More than 10 years, amortized cost	699,598
Amortized Cost	710,600	769,498
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	56
5 to 10 years, fair value	10,337
More than 10 years, fair value	568,443
Total, fair value	578,836	655,127
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Amortized Cost
Within one year, amortized cost	29,718
1 to 5 years, amortized cost	725,148
5 to 10 years, amortized cost	207,265
More than 10 years, amortized cost	61,533
Amortized Cost	1,023,664	877,590
Fair Value
Within one year, fair value	28,977
1 to 5 years, fair value	682,703
5 to 10 years, fair value	171,393
More than 10 years, fair value	55,662
Total, fair value	938,735	805,945
Corporate debt securities and other debt securities
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	8,984
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	8,984	8,908
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	8,648
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	$ 8,648	$ 8,601

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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