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, 2020	Dec. 31, 2019
Amortized Cost
Within One year, Amortized Cost	$ 20,684
1 to 5 Years, Amortized Cost	147,563
5 to 10 Years, Amortized Cost	321,871
More Than 10 years, Amortized Cost	6,905,147
Amortized Cost	7,395,265	$ 6,647,791
Fair Value
Within One Year, Fair Value	20,692
1 to 5 Years, Fair Value	154,223
5 to 10 Years, Fair Value	333,954
More Than 10 years, Fair Value	7,057,656
Total, Fair Value	7,566,525	6,778,670
U.S. Treasury securities
Amortized Cost
Within One year, Amortized Cost	20,254
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
Amortized Cost	20,254	19,855
Fair Value
Within One Year, Fair Value	20,254
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
Total, Fair Value	20,254	19,855
U.S. Government agency securities
Amortized Cost
Within One year, Amortized Cost	430
1 to 5 Years, Amortized Cost	26,432
5 to 10 Years, Amortized Cost	127,122
More Than 10 years, Amortized Cost	0
Amortized Cost	153,984	35,499
Fair Value
Within One Year, Fair Value	438
1 to 5 Years, Fair Value	26,243
5 to 10 Years, Fair Value	129,933
More Than 10 years, Fair Value	0
Total, Fair Value	156,614	36,541
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	1,585
5 to 10 Years, Amortized Cost	311
More Than 10 years, Amortized Cost	1,130,920
Amortized Cost	1,132,816	56,328
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	1,641
5 to 10 Years, Fair Value	324
More Than 10 years, Fair Value	1,127,427
Total, Fair Value	1,129,392	56,816
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	310
5 to 10 Years, Amortized Cost	73,420
More Than 10 years, Amortized Cost	4,328,321
Amortized Cost	4,402,051	5,079,396
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	319
5 to 10 Years, Fair Value	76,148
More Than 10 years, Fair Value	4,463,919
Total, Fair Value	4,540,386	5,180,815
Collateralized 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	248
More Than 10 years, Amortized Cost	1,330,763
Amortized Cost	1,331,011
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	259
More Than 10 years, Fair Value	1,345,025
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	109,731
5 to 10 Years, Amortized Cost	112,089
More Than 10 years, Amortized Cost	112,643
Amortized Cost	334,463	357,291
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	116,705
5 to 10 Years, Fair Value	118,468
More Than 10 years, Fair Value	118,984
Total, Fair Value	354,157	371,592
State and municipal securities
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	500
Amortized Cost	500	2,069
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	501
Total, Fair Value	501	2,075
Corporate debt securities and other debt securities
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	9,505
5 to 10 Years, Amortized Cost	8,681
More Than 10 years, Amortized Cost	2,000
Amortized Cost	20,186	144,410
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	9,315
5 to 10 Years, Fair Value	8,822
More Than 10 years, Fair Value	1,800
Total, Fair Value	19,937	146,725
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Amortized Cost
Amortized Cost	1,331,011	629,706
Fair Value
Total, Fair Value	$ 1,345,284	$ 636,851

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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