Principal Payments Due on Long-Term Borrowings (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
2017	$ 300.0
2019	450.0
2020	500.0
Thereafter	2,127.4
Total principal payment	3,377.4	$ 3,393.1
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
2017	300.0
Total principal payment	300.0	300.0
2010 Senior Notes [Member]
Debt Instrument [Line Items]
2020	500.0
Total principal payment	500.0	500.0
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Thereafter	500.0
Total principal payment	500.0	500.0
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Thereafter	500.0
Total principal payment	500.0	500.0
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
2019	450.0
Total principal payment	450.0	450.0
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Thereafter	600.0
Total principal payment	600.0	600.0
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Thereafter	527.4
Total principal payment	$ 527.4	$ 543.1

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 470
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-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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