Retirement benefits (Summary of Contribution Expense) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
401(k) contribution expense	$ 396	$ 1,267	$ 1,509	$ 1,543

Amount of cost for defined contribution plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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