Number of shares that have been repurchased during the period and are being held in treasury.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: 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(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Share Repurchases - Additional Information (Details) - USD ($) shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Nov. 02, 2016
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount			$ 500.0
Stock purchases, shares	6.0
Payment for share repurchase	$ 300.0	$ 200.0
Evergreen Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase	125.0
Repurchase Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase	175.0
Remaining amount for share repurchase program	$ 125.0