Long-Term Debt - 2015 Credit Agreement (Details) - Loans Payable - USD ($)	12 Months Ended
Dec. 31, 2018	May 05, 2015
2015 Term Loan Facility
Debt Instrument [Line Items]
Face amount of debt		$ 200,000,000
Term of debt	5 years
Revolving Credit Facility \| 2015 Revolving Credit Facility
Debt Instrument [Line Items]
Face amount of debt		$ 400,000,000

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