Summary of Total Indebtedness (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Fair value of interest rate swap	$ 12.6	$ 41.8
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	6.06%
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 1.7	2.0
Fair value of interest rate swap	$ 9.4	5.8
Notes Payable, interest rate	5.50%
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 2.8	3.1
Notes Payable, interest rate	4.50%
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 2.3	2.5
Notes Payable, interest rate	4.875%
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 0.5	0.7
Fair value of interest rate swap	$ 2.3	1.4
Notes Payable, interest rate	2.75%
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 1.6	$ 1.6
Notes Payable, interest rate	5.25%
Unamortized premium on issuance	$ 5.0
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable, interest rate	1.75%

Contractual interest rate for funds borrowed, under the debt agreement.

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(a)(1))
-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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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