Long-Term Debt - 5.25% Senior Notes (Details) - 5.25% Senior Note - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 08, 2015
Debt Instrument [Line Items]
Interest rate	5.25%		5.25%
Face amount	$ 300,000,000	$ 300,000,000	$ 300,000,000
Unamortized underwriters' fees, discount and debt issuance costs	$ 3,300,000

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