Securities, AFS Debt Securities, Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Amortized Cost [Abstract]
Within one year	$ 473
From one to five years	17,006
From five to ten years	146,941
After ten years	808,438
Amortized cost	972,858	$ 968,554
Available-for-sale Securities, Debt Maturities, Estimated Fair Value [Abstract]
Within one year	484
From one to five years	17,501
From five to ten years	151,270
After ten years	831,725
Fair value	$ 1,000,980	$ 975,340