Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized loss position for less than 12 months
Fair Value	$ 205,313	$ 879,683
Unrealized Loss	(473)	(2,260)
Unrealized loss position for more than 12 months
Fair Value	429,414	464,035
Unrealized Loss	$ (3,283)	$ (3,286)
Investment securities
Unrealized loss position for less than 12 months
Number of securities in loss position | security	27	57
Unrealized loss position for more than 12 months
Number of securities in loss position | security	63	62
Floating Interest Rate | Floating rate auction-rate certificates backed by Government guaranteed student loans
Unrealized loss position for less than 12 months
Fair Value	$ 0	$ 0
Unrealized Loss	0	0
Unrealized loss position for more than 12 months
Fair Value	19,072	18,912
Unrealized Loss	(591)	(788)
Floating Interest Rate | Floating rate asset-backed securities
Unrealized loss position for less than 12 months
Fair Value	0	2,583
Unrealized Loss	0	(1)
Unrealized loss position for more than 12 months
Fair Value	6,873	8,502
Unrealized Loss	(4)	(6)
Floating Interest Rate | Guaranteed mortgage-backed securities
Unrealized loss position for less than 12 months
Fair Value	154,452	841,993
Unrealized Loss	(464)	(2,244)
Unrealized loss position for more than 12 months
Fair Value	403,469	436,621
Unrealized Loss	(2,688)	(2,492)
Fixed Interest Rate | Fixed rate U.S. Treasuries
Unrealized loss position for less than 12 months
Fair Value	50,861	35,107
Unrealized Loss	(9)	(15)
Unrealized loss position for more than 12 months
Fair Value	0	0
Unrealized Loss	$ 0	$ 0