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

Debt (Details) - USD ($) $ in Millions	Jun. 30, 2017	Apr. 30, 2017	Dec. 31, 2016
Senior Notes 6 Percent Due April 15, 2017
Debt
Debt, interest rate (as a percent)		6.00%
Debt, face amount		$ 200
Senior Notes 1.8 Percent Due September 25, 2017
Debt
Debt, interest rate (as a percent)	1.80%		1.80%
Debt, face amount	$ 300