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Fair value measurements (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Carrying amount [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 2,189,986	$ 2,094,727	$ 2,105,832
Fair value [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 2,340,681	$ 2,239,445	$ 2,186,839

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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