Pension Plan and Employee Benefits - 401 (K) Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
401(k) Plan expense	$ 9,685	$ 7,861	$ 5,884
Support Group
Defined Contribution Plan Disclosure [Line Items]
401(k) Plan expense	$ 4,424	$ 3,408	$ 1,066