Benefit Plans - Projected Benefit Obligation, Accumulated Benefit Obligation and Fair Value of Plan Assets for Non-Qualified Plans (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 190,849	$ 175,405	$ 157,364
Accumulated benefit obligation	190,849	175,405
Fair value of plan assets	260,651	236,621	$ 210,508
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	20,513	20,081
Accumulated benefit obligation	20,513	20,081
Fair value of plan assets	$ 0	$ 0