Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Amortized Cost
Total	$ 0
Fair Value
Fair Value	1,915,461,000
Amortized Cost
Due in one year or less	12,015,000
Due after one year through five years	36,369,000
Due after five years through ten years	415,577,000
Due after ten years	1,451,500,000
Fair Value	1,915,461,000	$ 1,940,763,000	$ 1,472,538,000
Fair Value
Due in one year or less	12,051,000
Due after one year through five years	37,250,000
Due after five years through ten years	427,830,000
Due after ten years	1,494,064,000
Fair Value	$ 1,971,195,000	$ 1,956,047,000	$ 1,466,249,000