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Fair value measurements Insurance contracts (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Fair Value Disclosures [Abstract]          
Investments used to satisfy nonqualified benefit plans obligations $ 72,800,000 $ 66,500,000 $ 72,800,000 $ 66,500,000 $ 67,500,000
Net unrealized gain (loss) of investments used to satisfy obligations under nonqualified benefit plans $ 1,400,000 $ (1,700,000) $ 5,300,000 $ 700,000