Derivative Instruments - Derivative Instruments Interest Rate Swap Agreement (Detail) (USD $) In Millions, unless otherwise specified		9 Months Ended
	Jan. 18, 2012	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.351% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.347% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.350% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.068% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.150% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.750% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.264% [Member]	Sep. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.814% [Member]
Derivative [Line Items]
Derivative, Effective Date		Apr. 25, 2011	Apr. 25, 2011	Apr. 25, 2011	Jan. 25, 2011	Apr. 25, 2011	Jan. 25, 2011	Apr. 25, 2011	Jan. 25, 2011
Derivative, Notional Amount		$ 250.0	$ 250.0	$ 250.0	$ 1,000.0	$ 1,000.0	$ 1,000.0	$ 1,000.0	$ 1,000.0
Derivative, Fixed Rate Paid	0.165%	1.351%	1.347%	1.35%	3.068%	3.15%	3.75%	3.264%	3.814%
Derivative, Variable Rate Received as of September 30, 2012		0.217%	0.217%	0.217%	0.217%	0.217%	0.217%	0.217%	0.217%
Derivative, Next Reset Date		Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012	Oct. 25, 2012
Derivative, Maturity Date		Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015