Postretirement Benefits - Pension Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair value of plan assets:
Fair value of plan assets, beginning of year	$ 743,900	$ 592,544
Actual (loss) gain on plan assets	(38,360)	108,618
Employer contributions	80,061	61,102
Benefits paid	(18,032)	(16,184)
Settlements	0	(2,180)
Fair value of plan assets, end of year	$ 767,569	$ 743,900