Fair value measurements (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Carrying (reported) amount, fair value disclosure [Member]
Fair value, balance sheet grouping, financial statement captions [Line Items]
Long-term debt	$ 2,006,754	$ 1,744,975	$ 1,665,631
Estimate of fair value, fair value disclosure [Member]
Fair value, balance sheet grouping, financial statement captions [Line Items]
Long-term debt	$ 2,090,208	$ 1,888,135	$ 1,839,430