Fair value measurements Insurance contracts (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 78,300,000	$ 73,100,000	$ 78,300,000	$ 73,100,000	$ 77,400,000
Net unrealized gains (losses) on investments used to satisfy obligations under nonqualified benefit plans	$ 1,400,000	$ 2,100,000	$ 900,000	$ 5,000,000.0