Financial Instruments and Concentrations of Risk (Details 5) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Concentration of net assets outside of the U.S.
Cash and cash equivalents	$ 40,598	$ 79,351	$ 80,029	$ 73,613
Other current assets	20,017	14,011
Total assets	2,589,802	2,470,614
Current liabilities	391,662	499,214
Outside Of United States [Member]
Concentration of net assets outside of the U.S.
Cash and cash equivalents	31,920
Other current assets	109,270
Noncurrent assets	407,900
Total assets	549,090
Current liabilities	71,682
Noncurrent liabilities	95,828
Total liabilities	167,510
Net assets	381,580
CANADA
Concentration of net assets outside of the U.S.
Cash and cash equivalents	17,710
Other current assets	58,271
Noncurrent assets	215,386
Total assets	291,367
Current liabilities	37,713
Noncurrent liabilities	75,977
Total liabilities	113,690
Net assets	177,677
UNITED KINGDOM
Concentration of net assets outside of the U.S.
Cash and cash equivalents	957
Other current assets	1,854
Noncurrent assets	147,687
Total assets	150,498
Current liabilities	1,442
Noncurrent liabilities	18,619
Total liabilities	20,061
Net assets	130,437
MEXICO
Concentration of net assets outside of the U.S.
Cash and cash equivalents	13,253
Other current assets	49,145
Noncurrent assets	44,827
Total assets	107,225
Current liabilities	32,527
Noncurrent liabilities	1,232
Total liabilities	33,759
Net assets	$ 73,466