Income Taxes - Tax Deductions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Net purchases of T&L Bonds	$ 40,751	$ 59,500	$ (49,772)
T&L Bonds held as of end of year	$ 302,636	$ 261,885