Financial Instruments Foreign Assets and Liabilites (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration of net assets outside of the U.S.
Cash and cash equivalents	$ 93,699	$ 74,216	$ 58,096	$ 40,598
Other current assets	14,297	18,630
Total assets	5,376,817	3,074,972
Current liabilities	766,561	$ 488,355
Subsidiaries [Member] | CANADA
Concentration of net assets outside of the U.S.
Cash and cash equivalents	33,601
Other current assets	62,790
Noncurrent assets	427,259
Total assets	523,650
Current liabilities	64,056
Noncurrent liabilities	71,309
Total liabilities	135,365
Net assets	388,285
Subsidiaries [Member] | UNITED KINGDOM
Concentration of net assets outside of the U.S.
Cash and cash equivalents	13,185
Other current assets	3,150
Noncurrent assets	136,800
Total assets	153,135
Current liabilities	4,410
Noncurrent liabilities	13,016
Total liabilities	17,426
Net assets	135,709
Subsidiaries [Member] | MEXICO
Concentration of net assets outside of the U.S.
Cash and cash equivalents	9,581
Other current assets	35,044
Noncurrent assets	42,272
Total assets	86,897
Current liabilities	19,574
Noncurrent liabilities	716
Total liabilities	20,290
Net assets	66,607
Subsidiaries [Member] | Non-US [Member]
Concentration of net assets outside of the U.S.
Cash and cash equivalents	56,367
Other current assets	100,984
Noncurrent assets	606,331
Total assets	763,682
Current liabilities	88,040
Noncurrent liabilities	85,041
Total liabilities	173,081
Net assets	$ 590,601