Summary of Characteristics of Interest Rate Swaps (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative Instruments:
Weighted average fixed rate	2.16%	2.14%
Weighted average fixed rate	0.19%	0.20%
Forward Starting Pay Fixed Swaps [Member]
Derivative Instruments:
Notional amount	$ 3,100,000
Weighted average fixed rate	3.00%
Forward Starting Receive Fixed Swaps [Member]
Derivative Instruments:
Notional amount	$ 1,400,000
Weighted average fixed rate	1.18%