Investment Securities - Schedule Of Amortized Cost And Fair Value By Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Within 1 year	$ 0
After 1 year; within 5 years	0
After 5 years; within 10 years	10,000
After 10 years	0
Subtotal	10,000
Government agency issued MBS and CMO	0
Amortized Cost	10,000	$ 10,000
Fair Value
Within 1 year	0
After 1 year; within 5 years	0
After 5 years; within 10 years	9,756
After 10 years	0
Subtotal	9,756
Government agency issued MBS and CMO	0
Total	9,756	9,901
Amortized Cost
Within 1 year	15,188
After 1 year; within 5 years	154,460
After 5 years; within 10 years	0
After 10 years	24,151
Subtotal	193,799
Government agency issued MBS and CMO	4,581,427
Amortized Cost	4,775,226
Fair Value
Within 1 year	15,025
After 1 year; within 5 years	153,261
After 5 years; within 10 years	3,741
After 10 years	30,295
Subtotal	202,322
Government agency issued MBS and CMO	4,406,061
Total	$ 4,608,383	$ 5,170,255