Long-term Compensation Plans - Long Term Cash Bonus Plan (LTCB) (Details) - Deferred Bonus $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allotted bonus total amount	$ 6.5
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	36 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	62 months 15 days

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration