Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Carrying Amount	$ 3,428.6	$ 3,380.6
Estimated Fair Value	3,687.1	3,527.8
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	299.9	299.8
Estimated Fair Value	318.9	320.6
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	518.6	505.8
Estimated Fair Value	560.6	551.2
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	494.9	494.7
Estimated Fair Value	548.1	530.0
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	494.8	494.6
Estimated Fair Value	553.9	533.8
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	457.5	449.4
Estimated Fair Value	461.5	454.3
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Carrying Amount	597.3	597.2
Estimated Fair Value	668.3	617.7
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Carrying Amount	565.6	539.1
Estimated Fair Value	$ 575.8	$ 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=49121117&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=49121117&loc=d3e13476-108611

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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

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