Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized loss position for less than 12 months
Fair Value	$ 537,162	$ 879,683
Unrealized Loss	(636)	(2,260)
Unrealized loss position for more than 12 months
Fair Value	349,825	464,035
Unrealized Loss	$ (2,944)	$ (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	62	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	19,171	18,912
Unrealized Loss	(493)	(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	6,231	8,502
Unrealized Loss	(1)	(6)
Floating Interest Rate | Guaranteed mortgage-backed securities
Unrealized loss position for less than 12 months
Fair Value	172,842	841,993
Unrealized Loss	(593)	(2,244)
Unrealized loss position for more than 12 months
Fair Value	324,423	436,621
Unrealized Loss	(2,450)	(2,492)
Fixed Interest Rate | Fixed rate U.S. Treasuries
Unrealized loss position for less than 12 months
Fair Value	364,320	35,107
Unrealized Loss	(43)	(15)
Unrealized loss position for more than 12 months
Fair Value	0	0
Unrealized Loss	$ 0	$ 0