Borrowings - Interest Expense Amounts Related to Convertible Notes (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Disclosure [Abstract]
Interest cost related to contractual interest coupon	$ 15.5	$ 15.5	$ 15.5
Interest cost related to amortization of the discount	$ 12.2	$ 11.3	$ 10.5

The after-tax amount of interest recognized in the period associated with any convertible debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1505-109256

Name:	us-gaap_InterestOnConvertibleDebtNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration