Unsecured Senior Notes Schedule of Unsecured Senior Notes (Details) (USD $) In Thousands, unless otherwise specified			9 Months Ended														0 Months Ended	9 Months Ended		0 Months Ended	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 5.693% unsecured senior notes [Member]		Sep. 30, 2013 5.194% Unsecured Senior Notes [Member]		Sep. 30, 2013 5.967%t unsecured Senior Notes [Member]		Sep. 30, 2013 5.708% Unsecured Senior Notes [Member]		Sep. 30, 2013 4.289% Unsecured Senior Notes [Member]		Sep. 30, 2013 3.853% Unsecured Senior Notes [Member]		Sep. 30, 2013 3.954% Unsecured Senior Notes [Member]		Apr. 11, 2013 3.279% Unsecured Senior Notes [Member]	Sep. 30, 2013 3.279% Unsecured Senior Notes [Member]		Jun. 27, 2013 3.916%t Unsecured Senior Notes [Member]	Sep. 30, 2013 3.916%t Unsecured Senior Notes [Member]		Sep. 30, 2013 Senior Notes [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate			5.625%		5.00%		5.875%		5.625%		4.125%		3.70%		3.85%		3.125%	3.125%		3.80%	3.80%
Effective Percentage			5.693%	[1]	5.194%	[1]	5.967%	[1]	5.708%	[1]	4.289%	[1]	3.853%	[1]	3.954%	[1]	3.279%	3.279%	[1]	3.916%	3.916%	[1]
Principal Amount			$ 300,000		$ 250,000		$ 700,000		$ 700,000		$ 850,000		$ 850,000		$ 1,000,000			$ 500,000			$ 700,000		$ 5,850,000
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]	Sep. 01, 2023	Sep. 01, 2023	[2]	Feb. 01, 2024	Feb. 01, 2024	[2]
Unsecured senior notes, discount	14,576	10,472																					(14,576)
Unsecured senior notes	$ 5,835,424	$ 4,639,528																					$ 5,835,424

[1] Yield on issuance date including the effects of discounts on the notes and the amortization of financing costs.

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