Derivatives and other Hedging Instruments - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative [Line Items]
Ineffective losses recorded on a current basis in earnings	$ 212,000	$ (169,000)	$ 96,000	$ (338,000)
Estimated amount related to derivatives reclassified to interest expense during the next 12 months			113,831,000
Future cash flows for forecasted transactions, average period in months			28 months
Gain recognized due to change in fair value of contracts	5,485,000		5,536,000
Derivative, description of terms	The Company's total Futures Contracts designed to replicate interest rate swaps had a notional equivalent amount of $800.0 million at June 30, 2013 with a weighted average swap equivalent rate of 1.35% and a weighted average term of 56 months as of June 30, 2013.
Maximum leverage ratio the Company can reach before it could be in default of one of its derivative obligations			10 to 1
Eurodollar Futures Contracts
Derivative [Line Items]
Future cash flows for forecasted transactions, average period in months			56 months
Futures contracts - Asset Derivatives found in Other Assets	5,587,000		5,587,000
Weighted average swap equivalent rate	1.35%		1.35%
Fair value of derivatives in net liability position, which includes accrued interest but excludes any adjustment for nonperformance risk, related to agreements	0		0
Eurodollar Futures Contracts | Foreign exchange future
Derivative [Line Items]
Futures contracts notional amount	800,000		800,000
Reclassification out of accumulated other comprehensive income | Accumulated net gain (loss) from designated or qualifying cash flow hedges
Derivative [Line Items]
Effective hedge losses reclassified into earnings	$ 57,294,000	$ (59,168,000)	$ 51,385,000	$ (87,591,000)