Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt - Enterprise Notes (Details) - Securities purchase agreement - USD ($) $ in Millions		12 Months Ended
	Nov. 20, 2013	Dec. 31, 2017	Dec. 31, 2016	Feb. 15, 2011
Long-term debt
Percentage of preferred return		3.00%
Preferred return payments paid			$ 6.4
Series A Notes
Long-term debt
Debt issued				$ 80.0
Payment of debt	$ 80.0
Series B Notes
Long-term debt
Debt issued				$ 70.0