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, 2020	Dec. 31, 2019
Amortized Cost
Within One year, Amortized Cost	$ 44,538
1 to 5 Years, Amortized Cost	224,144
5 to 10 Years, Amortized Cost	350,039
More Than 10 years, Amortized Cost	6,379,634
Amortized Cost	6,998,355	$ 6,647,791
Fair Value
Within One Year, Fair Value	44,459
1 to 5 Years, Fair Value	230,196
5 to 10 Years, Fair Value	367,877
More Than 10 years, Fair Value	6,554,961
Total, Fair Value	7,197,493	6,778,670
U.S. Treasury securities
Amortized Cost
Within One year, Amortized Cost	19,921
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
Amortized Cost	19,921	19,855
Fair Value
Within One Year, Fair Value	19,921
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
Total, Fair Value	19,921
U.S. Government agency securities
Amortized Cost
Within One year, Amortized Cost	568
1 to 5 Years, Amortized Cost	27,714
5 to 10 Years, Amortized Cost	140,636
More Than 10 years, Amortized Cost	0
Amortized Cost	168,918	35,499
Fair Value
Within One Year, Fair Value	571
1 to 5 Years, Fair Value	27,770
5 to 10 Years, Fair Value	143,025
More Than 10 years, Fair Value	0
Total, Fair Value	171,366
Mortgage-backed securities issued by U.S. Government agencies
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	1,398
5 to 10 Years, Amortized Cost	715
More Than 10 years, Amortized Cost	858,660
Amortized Cost	860,773	56,328
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	1,442
5 to 10 Years, Fair Value	743
More Than 10 years, Fair Value	862,243
Total, Fair Value	864,428
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Amortized Cost
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	410
5 to 10 Years, Amortized Cost	83,168
More Than 10 years, Amortized Cost	4,209,985
Amortized Cost	4,293,563	5,079,396
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	423
5 to 10 Years, Fair Value	89,415
More Than 10 years, Fair Value	4,354,414
Total, Fair Value	4,444,252
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	0
5 to 10 Years, Amortized Cost	273
More Than 10 years, Amortized Cost	1,191,357
Amortized Cost	1,191,630
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	285
More Than 10 years, Fair Value	1,212,796
Total, Fair Value	1,213,081
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	109,753
5 to 10 Years, Amortized Cost	116,092
More Than 10 years, Amortized Cost	117,129
Amortized Cost	342,974	357,291
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	116,639
5 to 10 Years, Fair Value	125,274
More Than 10 years, Fair Value	123,340
Total, Fair Value	365,253	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	500
More Than 10 years, Amortized Cost	503
Amortized Cost	1,003	2,069
Fair Value
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	501
More Than 10 years, Fair Value	506
Total, Fair Value	1,007	2,075
Corporate debt securities and other debt securities
Amortized Cost
Within One year, Amortized Cost	24,049
1 to 5 Years, Amortized Cost	84,869
5 to 10 Years, Amortized Cost	8,655
More Than 10 years, Amortized Cost	2,000
Amortized Cost	119,573	144,410
Fair Value
Within One Year, Fair Value	23,967
1 to 5 Years, Fair Value	83,922
5 to 10 Years, Fair Value	8,634
More Than 10 years, Fair Value	1,662
Total, Fair Value	$ 118,185	$ 146,725

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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