Investments - Maturities of AFS Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Within one year	$ 78,206
After one, but within five years	258,270
After five, but within ten years	227,730
Greater than ten years	755,189
Amortized Cost	1,319,395	$ 1,198,513
Fair Value
Within one year	79,090
After one, but within five years	267,989
After five, but within ten years	237,588
Greater than ten years	754,741
Total	$ 1,339,408	$ 1,243,693