Unsecured Senior Notes (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012		Sep. 30, 2012
Proceeds from unsecured senior notes			$ 997,790,000
Amount paid for repurchase			225,000,000
Gains (losses) on early extinguishments of debt	(5,494,000)		(4,453,000)
11 Year Unsecured Senior Notes, 3.954% [Member]
Public offering date			Jun. 11, 2012
Aggregate principal amount	1,000,000,000		1,000,000,000
Interest at a fixed rate	3.85%		3.85%
Note price, percentage of principal amount			99.779%
Effective interest rate	3.954%	[1]	3.954%	[1]
Maturity Date			Feb. 01, 2023	[2]
Proceeds from unsecured senior notes			989,400,000
Senior Notes [Member]
Maximum leverage ratio			60.00%
Maximum secured debt leverage ratio			50.00%
Maximum interest coverage ratio			1.50
Minimum unencumbered asset value percentage of unsecured debt			150.00%
Senior Notes Six Point Two Five Percent Due Two Thousand Thirteen [Member]
Interest at a fixed rate	6.25%		6.25%
Redemption date			Aug. 24, 2012
Debt instrument repurchased	225,000,000		225,000,000
Amount paid for repurchase			231,600,000
Accrued and unpaid interest to, but not including, the redemption date			1,500,000
Redemption price			102.25%
Gains (losses) on early extinguishments of debt			(5,200,000)
Payment of redemption premium			$ 5,100,000

[1] Yield on issuance date including the effects of discounts on the notes.

[2] No principal amounts are due prior to maturity.