Unconditional Purchase Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unconditional Purchase Obligations (Excluding Capital Stock Redemptions) [Abstract]
Unconditional purchase obligations, beginning of year	$ 14,100	$ 7,200	$ 5,300
Future expenditures under purchase obligations, next twelve months	22,408
Future expenditures under purchase obligations, year two	18,271
Future expenditures under purchase obligations, year three	6,706
Future expenditures under purchase obligations, year four	4,545
Future expenditures under purchase obligations, year five	0
Future expenditures under purchase obligations	$ 51,930