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Fair value measurements Insurance contracts (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Fair Value Disclosures [Abstract]          
Investments used to satisfy nonqualified benefit plans obligations $ 75.0 $ 72.8 $ 75.0 $ 72.8 $ 70.9
Net unrealized gains on investments used to satisfy obligations under nonqualified benefit plans $ 1.9 $ 1.4 $ 6.9 $ 5.3