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, 2023	Dec. 31, 2022
Amortized Cost
Within one year, amortized cost	$ 24,818
1 to 5 years, amortized cost	1,080,485
5 to 10 years, amortized cost	402,201
More than 10 years, amortized cost	9,668,287
Amortized Cost	11,175,791	$ 11,273,327
Fair Value
Within one year, fair value	24,819
1 to 5 years, fair value	1,021,472
5 to 10 years, fair value	356,431
More than 10 years, fair value	8,329,896
Total, fair value	9,732,618	9,678,103
U.S. Treasury securities
Amortized Cost
Within one year, amortized cost	24,560
1 to 5 years, amortized cost	442,362
5 to 10 years, amortized cost	49,860
More than 10 years, amortized cost	0
Amortized Cost	516,782	515,953
Fair Value
Within one year, fair value	24,560
1 to 5 years, fair value	414,370
5 to 10 years, fair value	41,350
More than 10 years, fair value	0
Total, fair value	480,280	471,813
U.S. Government agency securities
Amortized Cost
Within one year, amortized cost	258
1 to 5 years, amortized cost	22,148
5 to 10 years, amortized cost	29,991
More than 10 years, amortized cost	0
Amortized Cost	52,397	52,411
Fair Value
Within one year, fair value	259
1 to 5 years, fair value	19,841
5 to 10 years, fair value	28,845
More than 10 years, fair value	0
Total, fair value	48,945	48,798
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	454
5 to 10 years, amortized cost	4
More than 10 years, amortized cost	809,584
Amortized Cost	810,042	904,593
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	441
5 to 10 years, fair value	4
More than 10 years, fair value	708,100
Total, fair value	708,545	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	20,872
5 to 10 years, amortized cost	87,576
More than 10 years, amortized cost	8,051,382
Amortized Cost	8,159,830	8,144,374
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	19,951
5 to 10 years, fair value	82,013
More than 10 years, fair value	6,923,536
Total, fair value	7,025,500	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	80
5 to 10 years, amortized cost	11,932
More than 10 years, amortized cost	741,865
Amortized Cost	753,877	769,498
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	77
5 to 10 years, fair value	11,605
More than 10 years, fair value	636,877
Total, fair value	648,559	655,127
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	585,636
5 to 10 years, amortized cost	222,838
More than 10 years, amortized cost	65,456
Amortized Cost	873,930	877,590
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	558,026
5 to 10 years, fair value	192,614
More than 10 years, fair value	61,383
Total, fair value	812,023	805,945
Corporate debt securities and other debt securities
Amortized Cost
Within one year, amortized cost	0
1 to 5 years, amortized cost	8,933
5 to 10 years, amortized cost	0
More than 10 years, amortized cost	0
Amortized Cost	8,933	8,908
Fair Value
Within one year, fair value	0
1 to 5 years, fair value	8,766
5 to 10 years, fair value	0
More than 10 years, fair value	0
Total, fair value	$ 8,766	$ 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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