Retirement plan expense (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Retirement Benefits [Abstract]
Defined contribution plan	$ 2.6	$ 2.2
Multiemployer plan	0.4	0.5
Retirement expense	$ 3.0	$ 2.7

The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.5,6,7)
-URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263

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