Supplemental Financial Information (Narrative) (Details)	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2013
Schedule Of Supplemental Financial Information [Line Items]
Ownership percentage	100.00%
Senior Subordinated 6.25% Notes
Schedule Of Supplemental Financial Information [Line Items]
Senior Subordinated Notes, interest rate	6.25%	6.25%	6.25%

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

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 210.5-02.22(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