Summary of Company's Interest Rate Derivatives (Detail) (Derivatives not designated as hedging instruments, USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Interest Rate Swap H, due August 2014
Derivatives, Fair Value [Line Items]
Notional Amount	$ 350.0		$ 350.0
Fair Value	(1.1)		(7.2)
Interest Rate Swap I, due August 2014
Derivatives, Fair Value [Line Items]
Notional Amount	350.0		350.0
Fair Value	(1.1)		(7.2)
Interest Rate Swap J, due August 2014
Derivatives, Fair Value [Line Items]
Notional Amount	125.0		125.0
Fair Value	(0.3)		(2.0)
Interest Rate Swap K, due August 2014
Derivatives, Fair Value [Line Items]
Notional Amount	125.0		125.0
Fair Value	(0.3)		(2.0)
Interest Rate Swap L, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	75.0	[1]	75.0	[1]
Fair Value	(1.7)	[1]	(0.4)	[1]
Interest Rate Swap M, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	100.0	[1]	100.0	[1]
Fair Value	(2.3)	[1]	(0.4)	[1]
Interest Rate Swap N, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	75.0	[1]	75.0	[1]
Fair Value	(1.6)	[1]	(0.2)	[1]
Interest Rate Swap O, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	75.0	[1]	75.0	[1]
Fair Value	(1.2)	[1]	0.2	[1]
Interest Rate Swap P, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	75.0	[1]	75.0	[1]
Fair Value	(0.9)	[1]	0.4	[1]
Interest Rate Swap Q, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	50.0	[1]	50.0	[1]
Fair Value	(0.5)	[1]	0.4	[1]
Interest Rate Swap R, due August 2019
Derivatives, Fair Value [Line Items]
Notional Amount	50.0	[1]	50.0	[1]
Fair Value	(0.5)	[1]	0.4	[1]
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional Amount	1,450.0		1,450.0
Fair Value	$ (11.5)		$ (18.0)

[1] include LIBOR floor of 1.00%