Other Assets - Claim Settlement Receivable (Details) - BP Exploration & Production Inc. - USD ($) $ in Millions	1 Months Ended	12 Months Ended
Oct. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Receivables
Litigation settlement amount received	$ 2.7
Interest income		$ 0.1	$ 0.1	$ 0.2

Cash received for the settlement of litigation during the current period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3536-108585

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