Unconditional Purchase Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unconditional Purchase Obligations (Excluding Capital Stock Redemptions) [Abstract]
Unconditional purchase obligations, beginning of year	$ 24,200	$ 22,400	$ 14,100
Future expenditures under purchase obligations, next twelve months	37,183
Future expenditures under purchase obligations, year two	14,034
Future expenditures under purchase obligations, year three	10,689
Future expenditures under purchase obligations, year four	6,212
Future expenditures under purchase obligations, year five	3,264
Future expenditures under purchase obligations	$ 71,382