Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2018 USD ($) position	Dec. 31, 2017 USD ($) position
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,433,453	$ 1,034,964
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	315,786	388,534
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3,281	3,909
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 2,647	$ 2,106
Investment Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | position	72	40
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	99.20%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | position	48	51
Floating Interest Rate [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans
Debt Securities, Available-for-sale, Unrealized Loss Position [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	18,715	18,814
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	985	886
Floating Interest Rate [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	6,456	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	19,058	23,145
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	90	120
Floating Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	927,416	292,522
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	196,416	221,641
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,907	2,337
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 1,360	$ 1,031
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	3.50%	2.50%
Fixed Interest Rate [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 499,581	$ 742,442
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	81,597	24,983
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	336	1,572
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 212	20
Fixed Interest Rate [Member] | USDA Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [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
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 49