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Investment Securities - Unrealized Loss Position (Details)
$ in Thousands
Sep. 30, 2018
USD ($)
position
Dec. 31, 2017
USD ($)
position
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 1,211,860 $ 1,034,964
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 353,693 388,534
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (2,449) (3,909)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses $ (2,093) $ (2,106)
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]    
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position 107 91
Investment Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]    
Available-for-sale securities in a loss position for more than 12 months, qualitative disclosure, percent of amortized cost 99.40%  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | position 45 51
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 0 $ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 18,961 18,814
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses (739) (886)
Floating Interest Rate [Member] | Asset-backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 20,080 23,145
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses (85) (120)
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 508,554 292,522
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 179,870 221,641
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (1,285) (2,337)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses $ (1,191) $ (1,031)
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield 3.30% 2.50%
Fixed Interest Rate [Member] | US Treasury Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 703,306 $ 742,442
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 134,782 24,983
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (1,164) (1,572)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses $ (78) (20)
Fixed Interest Rate [Member] | USDA Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value   0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value   99,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss   0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses   $ (49)