Derivative Instruments and Hedging Activities - Impact of Derivative Instruments on Consolidated Statements of Income and OCI (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Loss Reclassified from Accumulated OCI into Income (Effective Portion)	$ (5.7)	$ (5.7)	$ (5.7)
Treasury Locks, Net Of Tax
Derivative Instruments, Gain (Loss) [Line Items]
Net Loss Recognized in Accumulated OCI (Effective Portion)	$ (14.2)	$ (17.7)

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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Balance Type:	credit
Period Type:	duration