Income Taxes - Operating loss carryforwards (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Federal
Operating loss carryforwards
Operating loss carryforwards	$ 0.0	$ 0.0
State
Operating loss carryforwards
Operating loss carryforwards	$ 358.8	$ 391.7

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

Name:	us-gaap_OperatingLossCarryforwards
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant