Issuance of Common Stock (Details) (USD $)	3 Months Ended				0 Months Ended	1 Months Ended
	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013 5.375% notes, issued in March 2003 and repaid in March 2013	Jan. 22, 2013 ARCT	Mar. 31, 2013 Common stock
Issuance of common stock
Common stock issued to shareholders		207,631,143	207,485,073		45,573,144
Shares issued						17,250,000
Issuance price (in dollars per share)						$ 45.90
Common stock purchased by the underwriters (in shares)						2,250,000
Underwriting discounts and offering costs						$ 36,300,000
Weighted Average Stated Interest Rate (as a percent)				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	$ 755,495,000					$ 755,500,000
Common stock redeemed (in shares)					208,709