Unsecured Senior Notes Schedule of Unsecured Senior Notes (Details) (USD $) In Thousands, unless otherwise specified			0 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jun. 11, 2012 Eleven Year Unsecured Senior Notes Three Point Nine Five Four Percent		Dec. 31, 2012 Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member] Twelve Year Unsecured Senior Notes Five Point Six Nine Three Percent		Dec. 31, 2012 Senior Notes [Member] Twelve Year Unsecured Senior Notes Five Point One Nine Four Percent		Dec. 31, 2012 Senior Notes [Member] Ten Year Unsecured Senior Notes Five Point Nine Six Seven Percent		Dec. 31, 2012 Senior Notes [Member] Ten Year Unsecured Senior Notes Five Point Seven Zero Eight Percent		Dec. 31, 2012 Senior Notes [Member] Ten Year Unsecured Senior Notes Four Point Two Eight Nine Percent		Dec. 31, 2012 Senior Notes [Member] Seven Year Unsecured Senior Notes Three Point Eight Five Three Percent		Dec. 31, 2012 Senior Notes [Member] Eleven Year Unsecured Senior Notes Three Point Nine Five Four Percent
Debt Instrument [Line Items]
Coupon/Stated Rate			3.85%				5.625%		5.00%		5.875%		5.625%		4.125%		3.70%		3.85%
Effective Rate			3.954%	[1]			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				$ 4,650,000		$ 300,000		$ 250,000		$ 700,000		$ 700,000		$ 850,000		$ 850,000		$ 1,000,000
Maturity date			Feb. 01, 2023				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]
Net unamortized discount	(10,472)	(9,814)				(10,472)
Total	$ 4,639,528	$ 3,865,186

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

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