Derivative Instruments - Derivative Instruments Interest Rate Swap Agreement (Detail) (Interest rate swaps [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Interest Rate Swap Fixed Rate Paid 1.351% [Member]
Derivative [Line Items]
Derivative, Effective Date	Apr. 25, 2011
Derivative, Notional Amount	$ 250.0
Derivative, Fixed Rate Paid	1.351%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 1.347% [Member]
Derivative [Line Items]
Derivative, Effective Date	Apr. 25, 2011
Derivative, Notional Amount	250.0
Derivative, Fixed Rate Paid	1.347%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 1.350% [Member]
Derivative [Line Items]
Derivative, Effective Date	Apr. 25, 2011
Derivative, Notional Amount	250.0
Derivative, Fixed Rate Paid	1.35%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 3.068% [Member]
Derivative [Line Items]
Derivative, Effective Date	Jan. 25, 2011
Derivative, Notional Amount	1,000.0
Derivative, Fixed Rate Paid	3.068%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 3.150% [Member]
Derivative [Line Items]
Derivative, Effective Date	Apr. 25, 2011
Derivative, Notional Amount	1,000.0
Derivative, Fixed Rate Paid	3.15%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 3.750% [Member]
Derivative [Line Items]
Derivative, Effective Date	Jan. 25, 2011
Derivative, Notional Amount	1,000.0
Derivative, Fixed Rate Paid	3.75%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 3.264% [Member]
Derivative [Line Items]
Derivative, Effective Date	Apr. 25, 2011
Derivative, Notional Amount	1,000.0
Derivative, Fixed Rate Paid	3.264%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015
Interest Rate Swap Fixed Rate Paid 3.814% [Member]
Derivative [Line Items]
Derivative, Effective Date	Jan. 25, 2011
Derivative, Notional Amount	$ 1,000.0
Derivative, Fixed Rate Paid	3.814%
Derivative, Variable Rate Received as of Dec. 31, 2012	0.21%
Derivative, Next Reset Date	Jan. 25, 2013
Derivative, Maturity Date	Jan. 25, 2015