Financial Instruments and Concentrations of Risk (Details 5) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration of net assets outside of the U.S.
Cash and cash equivalents	$ 58,096	$ 40,598	$ 79,351	$ 80,029
Other current assets	19,387	20,017
Total assets	2,870,690	2,589,802
Current liabilities	377,027	$ 391,662
Outside Of United States [Member]
Concentration of net assets outside of the U.S.
Cash and cash equivalents	46,081
Other current assets	82,296
Noncurrent assets	458,988
Total assets	587,365
Current liabilities	62,069
Noncurrent liabilities	64,771
Total liabilities	126,840
Net assets	460,525
CANADA
Concentration of net assets outside of the U.S.
Cash and cash equivalents	20,729
Other current assets	47,267
Noncurrent assets	273,967
Total assets	341,963
Current liabilities	40,541
Noncurrent liabilities	47,971
Total liabilities	88,512
Net assets	253,451
UNITED KINGDOM
Concentration of net assets outside of the U.S.
Cash and cash equivalents	8,014
Other current assets	3,413
Noncurrent assets	144,367
Total assets	155,794
Current liabilities	5,660
Noncurrent liabilities	15,633
Total liabilities	21,293
Net assets	134,501
MEXICO
Concentration of net assets outside of the U.S.
Cash and cash equivalents	17,338
Other current assets	31,616
Noncurrent assets	40,654
Total assets	89,608
Current liabilities	15,868
Noncurrent liabilities	1,167
Total liabilities	17,035
Net assets	$ 72,573