Summary of Company's Interest Rate Derivatives (Detail) - Derivatives not designated as hedging instruments - USD ($)		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair Value		$ (19,000,000)	$ (29,000,000)
Interest Rate Swap L
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	75,000,000	75,000,000
Fair Value	[1]	(2,000,000)	(4,000,000)
Interest Rate Swap M
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	100,000,000	100,000,000
Fair Value	[1]	(3,000,000)	(5,000,000)
Interest Rate Swap N
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	75,000,000	75,000,000
Fair Value	[1]	(2,000,000)	(3,000,000)
Interest Rate Swap O
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	75,000,000	75,000,000
Fair Value	[1]	(2,000,000)	(3,000,000)
Interest Rate Swap P
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	75,000,000	75,000,000
Fair Value	[1]	(2,000,000)	(3,000,000)
Interest Rate Swap Q
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(2,000,000)
Interest Rate Swap R
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(2,000,000)
Interest Rate Swap S
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(1,000,000)
Interest Rate Swap T
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	75,000,000	75,000,000
Fair Value	[1]	(2,000,000)	(2,000,000)
Interest Rate Swap U
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(1,000,000)
Interest Rate Swap V
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(1,000,000)
Interest Rate Swap W
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	25,000,000	25,000,000
Fair Value	[1]		(1,000,000)
Interest Rate Swap X
Derivatives, Fair Value [Line Items]
Notional Amount	[1]	50,000,000	50,000,000
Fair Value	[1]	(1,000,000)	(1,000,000)
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount		$ 800,000,000	$ 800,000,000

[1] includes LIBOR floor of 1.00%