Investment Securities - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities available-for-sale
Less than 12 months, Fair Value	$ 4,252,125	$ 742,548
Less than 12 months, Unrealized Losses	143,187	4,289
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	4,252,125	742,548
Total, Unrealized Losses	143,187	4,289
Investment securities held-to-maturity
Less than 12 months, Fair Value	108,682
Less than 12 months, Unrealized Losses	3,982
12 months of more, Fair Value	0
12 months of more, Unrealized Losses	0
Total, Fair Value	108,682
Total, Unrealized Losses	3,982
Obligations of states and political subdivisions
Investment securities available-for-sale
Less than 12 months, Fair Value	130,928	78,241
Less than 12 months, Unrealized Losses	8,041	1,087
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	130,928	78,241
Total, Unrealized Losses	8,041	1,087
Government and government-sponsored enterprises
Investment securities available-for-sale
Less than 12 months, Fair Value	93,950	139,940
Less than 12 months, Unrealized Losses	649	813
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	93,950	139,940
Total, Unrealized Losses	649	813
Total mortgage-backed debt securities
Investment securities available-for-sale
Less than 12 months, Fair Value	4,027,247	524,367
Less than 12 months, Unrealized Losses	134,497	2,389
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	4,027,247	524,367
Total, Unrealized Losses	134,497	2,389
Residential agency
Investment securities available-for-sale
Less than 12 months, Fair Value	3,848,667	426,171
Less than 12 months, Unrealized Losses	123,937	1,388
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	3,848,667	426,171
Total, Unrealized Losses	123,937	1,388
Investment securities held-to-maturity
Less than 12 months, Fair Value	108,682
Less than 12 months, Unrealized Losses	3,982
12 months of more, Fair Value	0
12 months of more, Unrealized Losses	0
Total, Fair Value	108,682
Total, Unrealized Losses	3,982
Residential non-agency
Investment securities available-for-sale
Less than 12 months, Fair Value	3,001	1,529
Less than 12 months, Unrealized Losses	3	2
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	3,001	1,529
Total, Unrealized Losses	3	2
Commercial agency
Investment securities available-for-sale
Less than 12 months, Fair Value	175,579	96,667
Less than 12 months, Unrealized Losses	10,557	999
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	175,579	96,667
Total, Unrealized Losses	10,557	999
Commercial non-agency
Investment securities available-for-sale
Less than 12 months, Fair Value	0	0
Less than 12 months, Unrealized Losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	$ 0	$ 0

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