Investment Securities (Amortized Cost And Estimated Fair Value By Contractual Maturity Of Investment Securities Available For Sale) (Details) $ in Thousands	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	$ 21,468
1 to 5 Years, Amortized Cost	72,081
5 to 10 Years, Amortized Cost	624,240
More Than 10 years, Amortized Cost	3,110,882
No Stated Maturity, Amortized Cost	3,263
Total, Amortized Cost	3,831,934
Within One Year, Fair Value	21,484
1 to 5 Years, Fair Value	71,760
5 to 10 Years, Fair Value	620,318
More Than 10 years, Fair Value	3,066,272
No Stated Maturity, Fair Value	3,108
Total, Fair Value	3,782,942
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	18,756
1 to 5 Years, Amortized Cost	64,685
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	83,441
Within One Year, Fair Value	18,756
1 to 5 Years, Fair Value	64,135
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
No Stated Maturity, Fair Value	0
Total, Fair Value	82,891
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	999
1 to 5 Years, Amortized Cost	5,613
5 to 10 Years, Amortized Cost	5,477
More Than 10 years, Amortized Cost	0
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	12,089
Within One Year, Fair Value	1,012
1 to 5 Years, Fair Value	5,744
5 to 10 Years, Fair Value	5,675
More Than 10 years, Fair Value	0
No Stated Maturity, Fair Value	0
Total, Fair Value	12,431
Mortgage-Backed Securities Issued By U.S. Government Agencies [Member]
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	33,513
More Than 10 years, Amortized Cost	133,281
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	166,794
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	33,728
More Than 10 years, Fair Value	132,551
No Stated Maturity, Fair Value	0
Total, Fair Value	166,279
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	93
1 to 5 Years, Amortized Cost	1,603
5 to 10 Years, Amortized Cost	570,250
More Than 10 years, Amortized Cost	2,265,413
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	2,837,359
Within One Year, Fair Value	96
1 to 5 Years, Fair Value	1,700
5 to 10 Years, Fair Value	565,770
More Than 10 years, Fair Value	2,234,978
No Stated Maturity, Fair Value	0
Total, Fair Value	2,802,544
Collateralized Mortgage-Backed Securities Issued By U.S. Government Sponsored Enterprises [Member]
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	0
More Than 10 years, Amortized Cost	710,188
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	710,188
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	696,892
No Stated Maturity, Fair Value	0
Total, Fair Value	696,892
State And Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Within One year, Amortized Cost	1,620
1 to 5 Years, Amortized Cost	180
5 to 10 Years, Amortized Cost	0
More Than 10 years, Amortized Cost	0
No Stated Maturity, Amortized Cost	0
Total, Amortized Cost	1,800
Within One Year, Fair Value	1,620
1 to 5 Years, Fair Value	181
5 to 10 Years, Fair Value	0
More Than 10 years, Fair Value	0
No Stated Maturity, Fair Value	0
Total, Fair Value	1,801
Other Investments [Member]
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
No Stated Maturity, Amortized Cost	3,263
Total, Amortized Cost	20,263
Within One Year, Fair Value	0
1 to 5 Years, Fair Value	0
5 to 10 Years, Fair Value	15,145
More Than 10 years, Fair Value	1,851
No Stated Maturity, Fair Value	3,108
Total, Fair Value	$ 20,104