Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment Securities [Member]
Available-for-sale:
Amount Outstanding	$ 3,113,742	$ 2,226,409
Unamortized Premium/(Discount)	(2,396)	(3,489)
Amortized Cost	3,111,346	2,222,920
Unrealized Gains	4,535	860
Unrealized Losses	(4,249)	(5,928)
Fair Value	3,111,632	2,217,852
Total investment securities
Amount Outstanding	3,158,774	2,271,441
Amortized Cost	3,156,378	2,267,952
Unrealized Gains	5,034	1,422
Unrealized Losses	(4,249)	(5,928)
Fair Value	3,157,163	2,263,446
Available-for-sale Securities [Member] \| Investment Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	(2,396)	(3,489)
Floating Interest Rate [Member] \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale:
Amount Outstanding	19,700	19,700
Amortized Cost	19,700	19,700
Unrealized Gains	0	0
Unrealized Losses	(492)	(985)
Fair Value	19,208	18,715
Floating Interest Rate [Member] \| Asset-backed Securities [Member]
Available-for-sale:
Amount Outstanding	24,860	28,940
Amortized Cost	24,744	28,804
Unrealized Gains	0	2
Unrealized Losses	(175)	(128)
Fair Value	24,569	28,678
Floating Interest Rate [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Amount Outstanding	1,621,660	1,379,472
Amortized Cost	1,622,931	1,381,000
Unrealized Gains	2,807	721
Unrealized Losses	(3,500)	(4,267)
Fair Value	1,622,238	1,377,454
Floating Interest Rate [Member] \| Available-for-sale Securities [Member] \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale:
Unamortized Premium/(Discount)	0	0
Floating Interest Rate [Member] \| Available-for-sale Securities [Member] \| Asset-backed Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	(116)	(136)
Floating Interest Rate [Member] \| Available-for-sale Securities [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	1,271	1,528
Fixed Interest Rate [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Amount Outstanding	325	384
Amortized Cost	325	385
Unrealized Gains	27	18
Unrealized Losses	0	0
Fair Value	352	403
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Unrealized Gains	499	562
Unrealized Losses	0	0
Fair Value	$ 45,531	$ 45,594
Total investment securities
Held-to-maturity investment securities weighted average yield	3.80%	3.50%
Fixed Interest Rate [Member] \| US Treasury Securities [Member]
Available-for-sale:
Amount Outstanding	$ 1,447,197	$ 797,913
Amortized Cost	1,443,646	793,031
Unrealized Gains	1,701	119
Unrealized Losses	(82)	(548)
Fair Value	1,445,265	792,602
Fixed Interest Rate [Member] \| Available-for-sale Securities [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	0	1
Fixed Interest Rate [Member] \| Available-for-sale Securities [Member] \| US Treasury Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	(3,551)	(4,882)
Fixed Interest Rate [Member] \| Held-to-maturity Securities [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	$ 0	$ 0

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