Investment Securities (Amortized Cost And Estimated Fair Value By Contractual Maturity Of Investment Securities Available For Sale) (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	$ 21,376
1 to 5 Years, Amortized Cost	111,825
5 to 10 Years, Amortized Cost	643,783
More Than 10 years, Amortized Cost	3,326,454
Total, Amortized Cost	4,103,438
Within One Year, Fair Value	21,401
1 to 5 Years, Fair Value	110,244
5 to 10 Years, Fair Value	629,844
More Than 10 years, Fair Value	3,229,489
Total, Fair Value	3,990,978
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	18,924
1 to 5 Years, Amortized Cost	103,731
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
Total, Amortized Cost	122,655
Within One Year, Fair Value	18,924
1 to 5 Years, Fair Value	102,007
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
Total, Fair Value	120,931
U.S. Government agency securities
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	2,330
1 to 5 Years, Amortized Cost	6,437
5 to 10 Years, Amortized Cost	2,002
More Than 10 years, Amortized Cost	0
Total, Amortized Cost	10,769
Within One Year, Fair Value	2,355
1 to 5 Years, Fair Value	6,514
5 to 10 Years, Fair Value	2,028
More Than 10 years, Fair Value	0
Total, Fair Value	10,897
Mortgage-backed securities issued by U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	29,355
More Than 10 years, Amortized Cost	86,533
Total, Amortized Cost	115,888
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	28,993
More Than 10 years, Fair Value	83,875
Total, Fair Value	112,868
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	7
1 to 5 Years, Amortized Cost	1,657
5 to 10 Years, Amortized Cost	578,774
More Than 10 years, Amortized Cost	2,154,212
Total, Amortized Cost	2,734,650
Within One Year, Fair Value	7
1 to 5 Years, Fair Value	1,723
5 to 10 Years, Fair Value	565,331
More Than 10 years, Fair Value	2,096,939
Total, Fair Value	2,664,000
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	18,652
More Than 10 years, Amortized Cost	1,083,709
Total, Amortized Cost	1,102,361
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	18,218
More Than 10 years, Fair Value	1,046,823
Total, Fair Value	1,065,041
State and municipal securities
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	115
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
Total, Amortized Cost	115
Within One Year, Fair Value	115
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
Total, Fair Value	115
Corporate debt and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	0
1 to 5 Years, Amortized Cost	0
5 to 10 Years, Amortized Cost	15,000
More Than 10 years, Amortized Cost	2,000
Total, Amortized Cost	17,000
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	15,274
More Than 10 years, Fair Value	1,852
Total, Fair Value	$ 17,126