Postretirement Benefits - Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Projected benefit obligation:
Projected benefit obligation, beginning of year	$ 883,814	$ 1,272,654
Service cost for benefits earned	28,763	50,242	$ 53,041
Interest cost on benefit obligation	50,193	39,764	36,824
Plan amendments	583	1,620
Actuarial loss (gain)	65,041	(448,330)
Benefits paid	(34,840)	(32,136)
Projected benefit obligation, end of year	993,554	883,814	$ 1,272,654
Accumulated benefit obligation, end of year	847,143	$ 762,180
Projected benefit obligations increase	$ 109,700
Discount rate used to measure future benefit obligations (as a percent)	5.34%	5.67%