Postretirement Benefits Other Than Pensions - Funded Status of Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 299,208	$ 344,760
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	31,866	30,122
Service cost	679	586
Interest cost	941	960
Employee contributions	316	256
Actuarial (gain) loss	(1,959)	1,622
Benefits paid	(1,929)	(1,680)
Benefit obligation at end of year	29,914	31,866
Fair value of plan assets at end of year	0	0
Funded status	$ (29,914)	$ (31,866)