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Investment Securities - Schedule of Investment Securities (Details)
$ in Thousands
Jun. 30, 2018
USD ($)
position
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
position
Schedule of Available-for-sale and Trading Securities [Line Items]      
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position 100   91
Available-for-sale Securities [Abstract]      
Derivative, Notional Amount $ 10,176,118   $ 8,860,296
Investment Securities [Member]      
Schedule of Available-for-sale and Trading Securities [Line Items]      
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | position 44   51
Available-for-sale securities in a loss position for more than 12 months, qualitative disclosure, percent of amortized cost 99.30%    
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance $ 2,328,659   $ 2,214,588
Unamortized Premium/(Discount) (1,050)   2,365
Available-for-sale Securities, Amortized Cost Basis 2,327,609   2,216,953
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 1,298   4,467
Available-for-sale Securities, Unrealized Losses (4,309)   (6,015)
Available-for-sale Securities, Fair Value Disclosure 2,324,598   2,215,405
Marketable Securities, Amortized Cost Basis 2,372,641   2,261,985
Marketable Securities, Gross Unrealized Gains 2,129   4,999
Marketable Securities, Gross Unrealized Losses (4,309)   (6,015)
Marketable Securities Unpaid Principal Balance 2,373,691   2,259,620
Marketable Securities [Abstract]      
Marketable Securities 2,370,461   2,260,969
Available-for-sale Securities [Member] | Investment Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) (1,050)   2,365
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance 19,700   19,700
Available-for-sale Securities, Amortized Cost Basis 19,700   19,700
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0   0
Available-for-sale Securities, Unrealized Losses (690)   (886)
Available-for-sale Securities, Fair Value Disclosure 19,010   18,814
Floating Interest Rate [Member] | Asset-backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance 31,531   34,462
Available-for-sale Securities, Amortized Cost Basis 31,399   34,308
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 25   22
Available-for-sale Securities, Unrealized Losses (103)   (120)
Available-for-sale Securities, Fair Value Disclosure 31,321   34,210
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance 1,367,091   1,289,123
Available-for-sale Securities, Amortized Cost Basis 1,368,887   1,291,340
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 1,250   2,215
Available-for-sale Securities, Unrealized Losses (2,012)   (3,368)
Available-for-sale Securities, Fair Value Disclosure 1,368,125   1,290,187
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) 0   0
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Asset-backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) (132)   (154)
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) 1,796   2,217
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance 416   451
Available-for-sale Securities, Amortized Cost Basis 416   2,589
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 23   2,230
Available-for-sale Securities, Unrealized Losses 0   0
Available-for-sale Securities, Fair Value Disclosure 439   4,819
Derivative, Notional Amount   $ 143,100 143,700
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment 45,032   45,032
Held to Maturity; Unrealized Holding Gains 831   532
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss 0   0
Held-to-maturity Securities, Fair Value 45,863   45,564
Held-to-Maturity Unpaid Principal Balance 45,032   45,032
Fixed Interest Rate [Member] | USDA Securities [Member]      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance     100,000
Available-for-sale Securities, Amortized Cost Basis     100,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax     0
Available-for-sale Securities, Unrealized Losses     (49)
Available-for-sale Securities, Fair Value Disclosure     99,951
Fixed Interest Rate [Member] | US Treasury Securities [Member]      
Available-for-sale Securities [Abstract]      
Available-for-Sale Securities Unpaid Principal Balance 909,921   770,852
Available-for-sale Securities, Amortized Cost Basis 907,207   769,016
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 0   0
Available-for-sale Securities, Unrealized Losses (1,504)   (1,592)
Available-for-sale Securities, Fair Value Disclosure 905,703   767,424
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) 0   2,138
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | USDA Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount)     0
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | US Treasury Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) (2,714)   (1,836)
Fixed Interest Rate [Member] | Available-for-sale Securities [Member] | Interest-Only-Strip [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount)   $ 4,100 4,300
Fixed Interest Rate [Member] | Held-to-maturity Securities [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]      
Available-for-sale Securities [Abstract]      
Unamortized Premium/(Discount) $ 0   $ 0