Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Securities [Member]
Available-for-sale:
Amount Outstanding	$ 2,467,255	$ 2,226,409
Unamortized Premium/(Discount)	(4,280)	(3,489)
Amortized Cost	2,462,975	2,222,920
Unrealized Gains	1,354	860
Unrealized Losses	(6,351)	(5,928)
Fair Value	2,457,978	2,217,852
Total investment securities
Amount Outstanding	2,512,287	2,271,441
Amortized Cost	2,508,007	2,267,952
Unrealized Gains	2,280	1,422
Unrealized Losses	(6,351)	(5,928)
Fair Value	2,503,936	2,263,446
Available-for-sale Securities [Member] \| Investment Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	(4,280)	(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	(788)	(985)
Fair Value	18,912	18,715
Floating Interest Rate [Member] \| Asset-backed Securities [Member]
Available-for-sale:
Amount Outstanding	27,726	28,940
Amortized Cost	27,596	28,804
Unrealized Gains	0	2
Unrealized Losses	(145)	(128)
Fair Value	27,451	28,678
Floating Interest Rate [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Amount Outstanding	1,379,534	1,379,472
Amortized Cost	1,380,927	1,381,000
Unrealized Gains	626	721
Unrealized Losses	(5,316)	(4,267)
Fair Value	1,376,237	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)	(130)	(136)
Floating Interest Rate [Member] \| Available-for-sale Securities [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Unamortized Premium/(Discount)	1,393	1,528
Fixed Interest Rate [Member] \| Government/GSE Guaranteed Mortgage Backed Securities [Member]
Available-for-sale:
Amount Outstanding	370	384
Amortized Cost	370	385
Unrealized Gains	23	18
Unrealized Losses	0	0
Fair Value	393	403
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Unrealized Gains	926	562
Unrealized Losses	0	0
Fair Value	$ 45,958	$ 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	$ 1,039,925	$ 797,913
Amortized Cost	1,034,382	793,031
Unrealized Gains	705	119
Unrealized Losses	(102)	(548)
Fair Value	1,034,985	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)	(5,543)	(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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