Fair Value Disclosure - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Aggregate notional amount of outstanding Fair value hedge derivative instruments	$ 350.0	$ 350.0
Aggregate notional amount of outstanding Cash flow hedge derivative instruments	99.1	73.4
Expected pre tax reclassification of net losses from accumulated other comprehensive income to earnings in next twelve months	6.9
Expected after tax reclassification of net losses from accumulated other comprehensive income to earnings in next twelve months	4.4
Aggregate fair value of all derivative instruments with Net liability position	$ 1.3
Fair Value Hedges [Member]
Derivative [Line Items]
Maturity date range of hedge derivatives	2012-2015
No. of instruments, outstanding	3	3
Cash Flow Hedges [Member]
Derivative [Line Items]
Maturity date range of hedge derivatives	2012-2019
No. of instruments, outstanding	12	11