Issuance of Common Stock (Details) (USD $)	6 Months Ended				0 Months Ended	1 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Mar. 31, 2013 5.375% notes, issued in March 2003 and repaid in March 2013	Jan. 22, 2013 ARCT	Apr. 30, 2014 Common stock	Mar. 31, 2013 Common stock
Issuance of common stock
Common stock issued to shareholders	222,623,256		207,485,073		45,573,144
Shares issued						13,800,000	17,250,000
Common stock purchased by the underwriters (in shares)						1,800,000	2,250,000
Underwriting discounts and offering costs						$ 22,800,000	$ 36,700,000
Weighted Average Stated Interest Rate (as a percent)				5.375%
Total value of common shares issued					2,000,000,000
Net proceeds from issuance of common shares	$ 528,627,000	$ 755,136,000				$ 528,600,000	$ 755,100,000
Common stock redeemed (in shares)					208,709