Borrowings - Schedule of Short-term Borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Amount	$ 1,426,083	$ 617,363
Weighted-average Rate	0.26%	0.25%
FHLB advances
Short-term Debt [Line Items]
Amount	$ 1,200,000	$ 400,000
Weighted-average Rate	0.29%	0.33%
Collateralized Deposits
Short-term Debt [Line Items]
Amount	$ 223,695	$ 217,363
Weighted-average Rate	0.07%	0.11%
Line-of-Credit - TCF Financial Corporation | Line of Credit
Short-term Debt [Line Items]
Amount	$ 2,388	$ 0
Weighted-average Rate	2.45%	0.00%