Unconditional Purchase Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unconditional Purchase Obligations (Excluding Capital Stock Redemptions) [Abstract]
Unconditional purchase obligations, beginning of year	$ 7,200	$ 5,300	$ 2,900
Future expenditures under purchase obligations, next twelve months	14,134
Future expenditures under purchase obligations, year two	10,288
Future expenditures under purchase obligations, year three	9,724
Future expenditures under purchase obligations, year four	2,617
Future expenditures under purchase obligations, year five	652
Future expenditures under purchase obligations	$ 37,415