Benefit Plans and Other Postretirement Benefits, Defined contribution plans (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Benefit Plans and Other Postretirement Benefits
Contributions to U.S. defined contribution plans by the Company, maximum percentage of eligible compensation	5.00%
Matching contributions to U.S. defined contribution plans by the Company	$ 4.3	$ 3.2

Amount of cost for defined contribution plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration