Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Amortized Cost
Total	$ 0
Fair Value
Fair Value	3,498,087,000
Amortized Cost
Due in one year or less	7,308,000
Due after one year through five years	57,855,000
Due after five years through ten years	531,275,000
Due after ten years	2,901,649,000
Amortized Cost	3,498,087,000	$ 1,940,763,000	$ 1,789,509,000
Fair Value
Due in one year or less	7,339,000
Due after one year through five years	59,731,000
Due after five years through ten years	547,293,000
Due after ten years	2,947,566,000
Fair Value	$ 3,561,929,000	$ 1,956,047,000	$ 1,813,134,000