Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Carrying Amount	$ 3,401.0	$ 2,547.3
Estimated Fair Value	3,527.8	2,772.7
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	300.0	300.0
Estimated Fair Value	320.6	334.6
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	507.8	503.8
Estimated Fair Value	551.2	564.4
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	497.2	496.9
Estimated Fair Value	530.0	537.1
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	497.7	497.5
Estimated Fair Value	533.8	548.4
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	451.8	450.7
Estimated Fair Value	454.3	454.3
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	603.4	298.4
Estimated Fair Value	617.7	$ 333.9
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	543.1
Estimated Fair Value	$ 520.2

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13476-108611

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+ References

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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 SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

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