Derivative Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified		12 Months Ended				1 Months Ended	12 Months Ended						0 Months Ended	1 Months Ended						0 Months Ended	1 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Put Option [Member]	Dec. 31, 2012 CMBS [Member]	Jan. 18, 2012 Credit Facilities [Member]	Dec. 31, 2011 Credit Facilities [Member]	Dec. 31, 2011 Interest rate swaps [Member]	Dec. 31, 2012 Interest rate swaps [Member] interest_rate_swap_agreements	Dec. 31, 2011 Interest rate swaps [Member]	Dec. 31, 2010 Interest rate swaps [Member]	Jan. 18, 2012 Interest rate swaps [Member] interest_rate_swap_agreements	Dec. 09, 2011 Interest rate caps [Member]	Dec. 31, 2010 Interest rate caps [Member]	Jan. 18, 2012 Prior to amendment of terms [Member] Maximum [Member] months	Dec. 31, 2011 Prior to amendment of terms [Member] Maximum [Member] months	Jan. 18, 2012 Prior to amendment of terms [Member] Interest rate swaps [Member]	Dec. 31, 2011 Prior to amendment of terms [Member] Interest rate swaps [Member] interest_rate_swap_agreements	Jun. 30, 2011 Prior to amendment of terms [Member] Interest rate swaps [Member]	Jun. 30, 2011 Interest Rate Swap Fixed Rate Paid 4.172% [Member] [Member] Interest rate swaps [Member]	Jun. 30, 2011 Swaps that were split [Member] Interest rate swaps [Member] interest_rate_swap_agreements	Jan. 18, 2012 After amendment of terms [Member] Minimum [Member] months	Dec. 31, 2011 After amendment of terms [Member] Minimum [Member] months	Dec. 31, 2012 After amendment of terms [Member] Interest rate swaps [Member]	Jan. 18, 2012 After amendment of terms [Member] Interest rate swaps [Member]	Jun. 30, 2011 After amendment of terms [Member] Interest rate swaps [Member] interest_rate_swap_agreements	Dec. 31, 2012 Interest Rate Swap Fixed Rate Paid 4.276% [Member] Interest rate swaps [Member]	Dec. 31, 2012 Interest Rate Swap Fixed Rate Paid 4.263% [Member] Interest rate swaps [Member]	Dec. 31, 2012 Derivatives designated as hedging instruments [Member] Interest rate caps [Member]	Dec. 31, 2011 Derivatives designated as hedging instruments [Member] Interest rate caps [Member]	Dec. 31, 2010 Derivatives designated as hedging instruments [Member] Interest rate caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest rate swap agreements							8			3						2			2					4
Notional amount of interest rate swap agreements							$ 5,750.0				$ 517.7	$ 554.3			$ 1,000.0	$ 1,000.0	$ 1,000.0		$ 2,000.0					$ 1,000.0			$ 6,500.0		$ 525.0
Debt Instrument, Face Value	24,103.1			3,000.0	2,000.0
Derivative, Decrease in Fixed Interest Rate																						12	16.5
Interest Rate Cap Agreement Termination Date			Feb. 13, 2013
LIBOR and fixed payment rate																		4.172%							4.276%	4.263%
Deferred losses recorded in AOCL reclassified to interest expense						183.2
Deferred losses frozen in AOCL amortized to interet expense								51.2
Variable rate received on swaps changed, LIBOR, month													3	3						1	1
LIBOR cap rate											7.00%	5.00%															4.50%
Cash flow hedging instruments																											4,650.2
Increase (decrease) in interest expense							169.6	201.1	265.8																		20.9	20.9	19.2
Additional increase (decrease) to interest expense																											2.6
Aggregate cash payment		19.0
Excluding variable rate debt	$ 5,750.0
Interest rate swap agreements, represents total debt	25.00%
Fixed-rate debt represents total debt	75.00%