Postretirement Benefits - Pension Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair value of plan assets:
Fair value of plan assets, beginning of year	$ 907,625	$ 767,569
Actual gain on plan assets	195,713	182,002
Employer contributions	1,571	1,508
Benefits paid	(23,848)	(43,454)
Fair value of plan assets, end of year	$ 1,081,061	$ 907,625