Fair Value Measurements (Narrative) (Details) - Long-term Debt [Member] - 4.05% Senior Notes due January 2020 [Member]	1 Months Ended	6 Months Ended	12 Months Ended
Jan. 31, 2015	Jun. 30, 2018	Dec. 31, 2017	Jul. 31, 2016
Debt Instrument [Line Items]
Stated interest rate	4.05%	4.05%	4.05%	5.80%
Maturity date, year and month	2020-01	2020-01	2020-01

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant