Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total long-term debt	$ 3,124.5	$ 2,547.3
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Notes Payable	300.0	300.0
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	515.4	503.8
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	497.1	496.9
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	497.6	497.5
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Notes Payable	457.9	450.7
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Notes Payable	298.4	$ 298.4
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	$ 558.1

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

Reference 1: 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(a))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: 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

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

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