Issuance of Common Stock (Details) (USD $)	12 Months Ended					0 Months Ended	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 5.375% notes, issued in March 2003 and repaid in March 2013	Dec. 31, 2012 5.375% notes, issued in March 2003 and repaid in March 2013	Jan. 22, 2013 ARCT	Oct. 31, 2013 Common stock	Mar. 31, 2013 Common stock
Issuance of common stock
Common stock issued to shareholders	207,485,073	133,452,411				45,573,144
Shares issued							9,775,000	17,250,000
Issuance price (in dollars per share)							$ 40.63	$ 45.90
Common stock purchased by the underwriters (in shares)							1,275,000	2,250,000
Underwriting discounts and offering costs	$ 55,359,000	$ 13,773,000	$ 25,200,000				$ 18,700,000	$ 36,700,000
Weighted Average Stated Interest Rate (as a percent)				5.375%	5.375%
Closing price of common stock (in dollars per share)						$ 44.04
Total value of common shares issued						2,000,000,000
Net proceeds from issuance of common shares	$ 1,133,574,000		$ 489,236,000				$ 378,500,000	$ 755,100,000
Common stock redeemed (in shares)						208,709