Debt Securities - AFS Debt Securities in an Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Available-for-sale
Number of available-for-sale securities | security	175	46
Less than 12 months, fair value	$ 2,454,519	$ 145,530
Less than 12 months, unrealized loss	(27,870)	(783)
12 months or more, fair value	3,204	753
12 months or more, unrealized loss	(181)	(9)
Total, fair value	2,457,723	146,283
Total, unrealized loss	$ (28,051)	$ (792)
Held-to-maturity
Number of held-to-maturity securities | security	65
Less than 12 Months	$ 217,131
Less than 12 Months	(728)
12 Months or More	0
12 Months or More	0
Total, fair value	217,131
Total, unrealized loss	$ (728)
U.S. government and federal agency
Available-for-sale
Number of available-for-sale securities | security	24	18
Less than 12 months, fair value	$ 17,857	$ 13,814
Less than 12 months, unrealized loss	(343)	(258)
12 months or more, fair value	2,361	726
12 months or more, unrealized loss	(147)	(9)
Total, fair value	20,218	14,540
Total, unrealized loss	$ (490)	$ (267)
State and local governments
Available-for-sale
Number of available-for-sale securities | security	57	6
Less than 12 months, fair value	$ 36,748	$ 3,121
Less than 12 months, unrealized loss	(480)	(54)
12 months or more, fair value	816	0
12 months or more, unrealized loss	(33)	0
Total, fair value	37,564	3,121
Total, unrealized loss	$ (513)	$ (54)
Held-to-maturity
Number of held-to-maturity securities | security	65
Less than 12 Months	$ 217,131
Less than 12 Months	(728)
12 Months or More	0
12 Months or More	0
Total, fair value	217,131
Total, unrealized loss	$ (728)
Corporate bonds
Available-for-sale
Number of available-for-sale securities | security	1	3
Less than 12 months, fair value	$ 3,915	$ 5,500
Less than 12 months, unrealized loss	(85)	(8)
12 months or more, fair value	0	0
12 months or more, unrealized loss	0	0
Total, fair value	3,915	5,500
Total, unrealized loss	$ (85)	$ (8)
Residential mortgage-backed securities
Available-for-sale
Number of available-for-sale securities | security	80	14
Less than 12 months, fair value	$ 2,158,485	$ 2,354
Less than 12 months, unrealized loss	(22,475)	(4)
12 months or more, fair value	27	27
12 months or more, unrealized loss	(1)	0
Total, fair value	2,158,512	2,381
Total, unrealized loss	$ (22,476)	$ (4)
Commercial mortgage-backed securities
Available-for-sale
Number of available-for-sale securities | security	13	5
Less than 12 months, fair value	$ 237,514	$ 120,741
Less than 12 months, unrealized loss	(4,487)	(459)
12 months or more, fair value	0	0
12 months or more, unrealized loss	0	0
Total, fair value	237,514	120,741
Total, unrealized loss	$ (4,487)	$ (459)