Benefit Plans and Other Postretirement Benefits, Defined contribution plans (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 6.9	$ 5.0	$ 4.2

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