Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost
Available for Sale, Due in one year or less	$ 9,427
Available for Sale, Due after one year through five years	83,136
Available for Sale, Due after five years through ten years	343,084
Available for Sale, Due after ten years	90,327
Available for Sale, Total debt securities	525,975
Available for Sale, amortized cost	3,100,291	$ 3,251,950
Fair Value
Available for Sale, Due in one year or less	9,458
Available for Sale, Due after one year through five years	83,882
Available for Sale, Due after five years through ten years	360,051
Available for Sale, Due after ten years	97,240
Available for Sale, Total debt securities	550,630
Investment securities available for sale, at fair value	$ 3,149,773	3,262,586
Available for Sale, Ratio of Fair Value to Amortized Cost	101.60%
Amortized Cost
Held to Maturity, Due in one year or less	$ 33,745
Held to Maturity, Due after one year through five years	67,917
Held to Maturity, Due after five years through ten years	166,553
Held to Maturity, Due after ten years	1,168,722
Held to Maturity, Total Debt Securities	1,436,937
Held to Maturity, Amortized Cost	2,077,287	2,205,083
Fair Value
Held to Maturity, Due in one year or less	33,927
Held to Maturity, Due after one year through five years	69,887
Held to Maturity, Due after five years through ten years	175,189
Held to Maturity, Due after ten years	1,268,050
Held to Maturity, Securities, Debt Securities	1,547,053
Held to Maturity, Total debt securities at fair value	$ 2,209,505	2,276,465
Held to Maturity, Ratio of Fair Value to Amortized Cost	106.40%
SBA
Amortized Cost
Available for Sale, amortized cost	$ 9,619
Fair Value
Investment securities available for sale, at fair value	9,575
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
FFELP
Amortized Cost
Available for Sale, amortized cost	356,532	270,178
Fair Value
Investment securities available for sale, at fair value	340,474	263,693
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
FNMA/FHLMC \| Residential Related Securities
Amortized Cost
Available for Sale, amortized cost	813,458	131,158
Fair Value
Investment securities available for sale, at fair value	818,110	132,660
Amortized Cost
Held to Maturity, Amortized Cost	71,872	81,676
Fair Value
Held to Maturity, Total debt securities at fair value	75,271	83,420
FNMA/FHLMC \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	19,793	19,929
Fair Value
Investment securities available for sale, at fair value	22,061	21,728
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
GNMA \| Residential Related Securities
Amortized Cost
Available for Sale, amortized cost	592,338	982,941
Fair Value
Investment securities available for sale, at fair value	605,561	985,139
Amortized Cost
Held to Maturity, Amortized Cost	207,908	269,523
Fair Value
Held to Maturity, Total debt securities at fair value	214,920	270,296
GNMA \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	782,577	1,314,836
Fair Value
Investment securities available for sale, at fair value	803,362	1,310,207
Amortized Cost
Held to Maturity, Amortized Cost	360,570	434,317
Fair Value
Held to Maturity, Total debt securities at fair value	$ 372,261	$ 434,503

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