Postretirement Benefits - Pension Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair value of plan assets:
Fair value of plan assets, beginning of year	$ 767,569	$ 743,900
Actual gain (loss) on plan assets	182,002	(38,360)
Employer contributions	1,508	80,061
Benefits paid	(43,454)	(18,032)
Fair value of plan assets, end of year	$ 907,625	$ 767,569