Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized loss position for less than 12 months
Fair Value	$ 874,161	$ 879,683
Unrealized Loss	(6,217)	(2,260)
Unrealized loss position for more than 12 months
Fair Value	436,331	464,035
Unrealized Loss	$ (8,035)	$ (3,286)
Debt Securities
Unrealized loss position for less than 12 months
Number of securities in loss position | security	65	57
Unrealized loss position for more than 12 months
Number of securities in loss position | security	67	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	16,721	18,912
Unrealized Loss	(2,955)	(788)
Floating Interest Rate | Floating rate asset-backed securities
Unrealized loss position for less than 12 months
Fair Value	2,544	2,583
Unrealized Loss	(9)	(1)
Unrealized loss position for more than 12 months
Fair Value	7,959	8,502
Unrealized Loss	(9)	(6)
Floating Interest Rate | Government/GSE Guaranteed Mortgage Backed Securities
Unrealized loss position for less than 12 months
Fair Value	871,617	841,993
Unrealized Loss	(6,208)	(2,244)
Unrealized loss position for more than 12 months
Fair Value	411,651	436,621
Unrealized Loss	$ (5,071)	(2,492)
Fixed Interest Rate | Fixed rate U.S. Treasuries
Unrealized loss position for less than 12 months
Fair Value		35,107
Unrealized Loss		(15)
Unrealized loss position for more than 12 months
Fair Value		0
Unrealized Loss		$ 0