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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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

Long-Term Debt - Senior Notes (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Apr. 04, 2017	Jul. 28, 2014
Senior Notes 5.125 Percent Due 2022
Long-Term Debt
Face amount of debt issuance	$ 250.0			$ 250.0
Interest rate, as a percentage	5.125%	5.125%
Senior Notes 5.50 Percent Due 2025
Long-Term Debt
Face amount of debt issuance	$ 300.0		$ 300.0
Interest rate, as a percentage			5.50%