The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the 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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

Debt (2013 Credit Facility - Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Jun. 30, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Amount outstanding	$ 0
Adjusted debt capital structure, percentage	31.00%
Long-term Debt [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 1,934,000,000
Long-term Debt [Member] \| 2013 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		66,000,000	$ 2,000,000,000
Credit facility, maturity date	Dec. 01, 2018
Amount outstanding	$ 0
Debt percentage of adjusted book capital structure covenant	60.00%
Long-term Debt [Member] \| 2016 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		743,000,000
2016 Term Loan due December 2020 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 1,191,000,000