Subsequent Event (Details) - Subsequent Event - Electrum - Celaya $ in Millions	1 Months Ended
Aug. 31, 2018 USD ($)
Subsequent Events
Interest in joint venture (as a percent)	100.00%
Payment to acquire additional interest in joint venture	$ 3.0

The cash outflow associated with the investment in or advances to an entity in which the reporting entity shares control of the entity with another party or group.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

Name:	us-gaap_PaymentsToAcquireInterestInJointVenture
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration