Unsecured Senior Notes (Schedule Of Unsecured Senior Notes) (Detail) (USD $) In Thousands, unless otherwise specified			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 12 Year Unsecured Senior Notes, 5.693% [Member]		Sep. 30, 2012 12 Year Unsecured Senior Notes, 5.194% [Member]		Sep. 30, 2012 10 Year Unsecured Senior Notes, 5.967% [Member]		Sep. 30, 2012 10 Year Unsecured Senior Notes, 5.708% [Member]		Sep. 30, 2012 10 Year Unsecured Senior Notes, 4.289% [Member]		Sep. 30, 2012 7 Year Unsecured Senior Notes, 3.853% [Member]		Sep. 30, 2012 11 Year Unsecured Senior Notes, 3.954% [Member]		Sep. 30, 2012 Senior Notes [Member]
Debt [Line Items]
Coupon/Stated Rate			5.625%		5.00%		5.875%		5.625%		4.125%		3.70%		3.85%
Effective Rate			5.693%	[1]	5.194%	[1]	5.967%	[1]	5.708%	[1]	4.289%	[1]	3.853%	[1]	3.954%	[1]
Principal Amount	$ 1,197,500		$ 300,000		$ 250,000		$ 700,000		$ 700,000		$ 850,000		$ 850,000		$ 1,000,000		$ 4,650,000
Net unamortized discount	(10,783)	(9,814)															(10,783)
Unsecured senior notes	$ 4,639,217	$ 3,865,186
Maturity Date			Apr. 15, 2015	[2]	Jun. 01, 2015	[2]	Oct. 15, 2019	[2]	Nov. 15, 2020	[2]	May 15, 2021	[2]	Nov. 15, 2018	[2]	Feb. 01, 2023	[2]

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

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