Derivatives - Fair Value of Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value at period end	$ (1.0)	[1]	$ (9.7)	[1]
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	60.0
Fair Value at period end			(0.2)	[1]
Interest Rate	1.74%
Maturity Date	Mar. 11, 2013
Interest Rate Swaps Two [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	200.0
Fair Value at period end			(1.0)	[1]
Interest Rate	2.96%
Maturity Date	Mar. 11, 2013
Interest Rate Swaps Three [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	40.0
Fair Value at period end			(0.1)	[1]
Interest Rate	1.82%
Maturity Date	Mar. 11, 2013
Interest Rate Swaps Four [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	65.0
Fair Value at period end			(1.5)	[1]
Interest Rate	3.60%
Maturity Date	Sep. 09, 2013
Interest Rate Swaps Five [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	68.0
Fair Value at period end			(1.6)	[1]
Interest Rate	3.69%
Maturity Date	Sep. 09, 2013
Interest Rate Swaps Six [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	35.0
Fair Value at period end			(1.4)	[1]
Interest Rate	3.73%
Maturity Date	Sep. 09, 2013
Interest Rate Swaps Seven [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	55.0
Fair Value at period end	(0.2)	[1]	(1.3)	[1]
Interest Rate	2.90%
Maturity Date	Nov. 29, 2013
Interest Rate Swaps Eight [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	48.0
Fair Value at period end	(0.2)	[1]	(1.2)	[1]
Interest Rate	2.90%
Maturity Date	Nov. 29, 2013
Interest Rate Swaps Nine [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	25.0
Fair Value at period end	(0.4)	[1]	(0.5)	[1]
Interest Rate	1.10%
Maturity Date	Jul. 31, 2016
Interest Rate Swap Ten [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	28.1
Fair Value at period end	(0.6)	[1]	(0.9)	[1]
Interest Rate	1.38%
Maturity Date	Jan. 02, 2017
Interest Rate Swap Eleven [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	35.1
Fair Value at period end	0.1	[1]
Interest Rate	3.72%
Maturity Date	Dec. 01, 2017
Interest Rate Swap Twelve [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	7.6
Fair Value at period end	0.1	[1]
Interest Rate	1.00%
Maturity Date	Jan. 01, 2018
Forward Starting Interest Rate Swaps One [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	48.0
Fair Value at period end	0.1	[1]
Interest Rate	1.12%
Effective Date	Dec. 01, 2013
Maturity Date	Jan. 01, 2018
Forward Starting Interest Rate Swaps Two [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(in millions of dollars) Notional Value	55.0
Fair Value at period end	$ 0.1	[1]
Interest Rate	1.12%
Effective Date	Dec. 01, 2013
Maturity Date	Jan. 01, 2018

[1] As of September 30, 2013 and December 31, 2012, derivative valuations in their entirety were classified in Level 2 of the fair value hierarchy. As of September 30, 2013 and December 31, 2012, we did not have any significant recurring fair value measurements related to derivative instruments using significant unobservable inputs (Level 3).