Commitments and Contingencies Commitments and Contingencies - Narrative -Standby Letters of Credit (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Revolving Credit Facility
Loss Contingencies [Line Items]
Outstanding standby letters of credit	$ 35.7	$ 35.1
Revolver commitments	$ 600.0	$ 600.0
Revolving Credit Facility \| Maximum [Member]
Loss Contingencies [Line Items]
Letter of Credit Sublimit	90.00%	90.00%
Letter of Credit
Loss Contingencies [Line Items]
Revolver commitments	$ 300.0	$ 300.0

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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