Issuance of Common Stock (Details) (USD $)	6 Months Ended				0 Months Ended	1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 5.375% notes, issued in March 2003 and due in March 2013	Dec. 31, 2012 5.375% notes, issued in March 2003 and due in March 2013	Jan. 22, 2013 ARCT	Mar. 31, 2013 Common stock
Issuance of common stock
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						$ 36,600,000
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	$ 755,136,000					$ 755,100,000
Common stock redeemed (in shares)					208,709
Common stock issued to shareholders	196,265,884	133,452,411			45,573,144