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Fair value measurements (Narratives) (Details) - USD ($)	Jun. 30, 2016	May 13, 2016	Dec. 31, 2015
Financial Liabilities
Liabilities fair value			$ 0
Significant transfers	$ 0
Capital lease obligations		$ 80,000,000
Restricted investment	69,100,000		$ 0
Reported Value
Financial Liabilities
Capital lease obligations	80,000,000
Debt fair value	486,600,000
Estimate of Fair Value
Financial Liabilities
Capital lease obligations	93,200,000
Level 1 \| Fair Value, Measurements, Recurring
Financial Liabilities
Restricted investment	69,100,000
Restricted investments, fair value	$ 70,200,000