Debt Securities - AFS Debt Securities in an Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Available-for-sale
Number of available-for-sale securities | security	148	46
Less than 12 months, fair value	$ 3,222,117	$ 145,530
Less than 12 months, unrealized loss	(19,194)	(783)
12 months or more, fair value	6,789	753
12 months or more, unrealized loss	(259)	(9)
Total, fair value	3,228,906	146,283
Total, unrealized loss	$ (19,453)	$ (792)
Held-to-maturity
Number of held-to-maturity securities | security	61
Less than 12 Months	$ 79,147
Less than 12 Months	(259)
12 Months or More	0
12 Months or More	0
Total, fair value	79,147
Total, unrealized loss	$ (259)
U.S. government and federal agency
Available-for-sale
Number of available-for-sale securities | security	23	18
Less than 12 months, fair value	$ 8,991	$ 13,814
Less than 12 months, unrealized loss	(47)	(258)
12 months or more, fair value	6,789	726
12 months or more, unrealized loss	(259)	(9)
Total, fair value	15,780	14,540
Total, unrealized loss	$ (306)	$ (267)
U.S. government sponsored enterprises
Available-for-sale
Number of available-for-sale securities | security	3
Less than 12 months, fair value	$ 45,234
Less than 12 months, unrealized loss	(298)
12 months or more, fair value	0
12 months or more, unrealized loss	0
Total, fair value	45,234
Total, unrealized loss	$ (298)
State and local governments
Available-for-sale
Number of available-for-sale securities | security	11	6
Less than 12 months, fair value	$ 17,169	$ 3,121
Less than 12 months, unrealized loss	(141)	(54)
12 months or more, fair value	0	0
12 months or more, unrealized loss	0	0
Total, fair value	17,169	3,121
Total, unrealized loss	$ (141)	$ (54)
Held-to-maturity
Number of held-to-maturity securities | security	61
Less than 12 Months	$ 79,147
Less than 12 Months	(259)
12 Months or More	0
12 Months or More	0
Total, fair value	79,147
Total, unrealized loss	$ (259)
Corporate bonds
Available-for-sale
Number of available-for-sale securities | security	1	3
Less than 12 months, fair value	$ 10,005	$ 5,500
Less than 12 months, unrealized loss	(4)	(8)
12 months or more, fair value	0	0
12 months or more, unrealized loss	0	0
Total, fair value	10,005	5,500
Total, unrealized loss	$ (4)	$ (8)
Residential mortgage-backed securities
Available-for-sale
Number of available-for-sale securities | security	92	14
Less than 12 months, fair value	$ 2,903,724	$ 2,354
Less than 12 months, unrealized loss	(14,673)	(4)
12 months or more, fair value	0	27
12 months or more, unrealized loss	0	0
Total, fair value	2,903,724	2,381
Total, unrealized loss	$ (14,673)	$ (4)
Commercial mortgage-backed securities
Available-for-sale
Number of available-for-sale securities | security	18	5
Less than 12 months, fair value	$ 236,994	$ 120,741
Less than 12 months, unrealized loss	(4,031)	(459)
12 months or more, fair value	0	0
12 months or more, unrealized loss	0	0
Total, fair value	236,994	120,741
Total, unrealized loss	$ (4,031)	$ (459)