Fair value measurements Insurance contracts (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Investments used to satisfy nonqualified benefit plans obligations	$ 83,100,000	$ 78,300,000	$ 83,100,000	$ 78,300,000	$ 73,800,000
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	$ 2,900,000	$ 1,400,000	$ 9,300,000	$ 900,000