Issuance of Common Stock (Detail) (USD $)	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 22, 2013	Apr. 30, 2014	Oct. 31, 2013	Mar. 31, 2013
Issuance of common stock
Common stock issued to shareholders	224,881,192	207,485,073
Underwriting discounts and offering costs	$ 22,827,000	$ 55,359,000	$ 13,773,000
Net proceeds from issuance of common shares	528,615,000	1,133,574,000
5.375% notes, issued in March 2003 and repaid in March 2013
Issuance of common stock
Weighted Average Stated Interest Rate (as a percent)							5.375%
ARCT
Issuance of common stock
Common stock issued to shareholders				45,573,144
Total value of common shares issued				2,000,000,000
Common stock redeemed (in shares)				208,709
Common stock
Issuance of common stock
Shares issued					13,800,000	9,775,000	17,250,000
Common stock purchased by the underwriters (in shares)					1,800,000	1,275,000	2,250,000
Underwriting discounts and offering costs					22,800,000	18,700,000	36,700,000
Net proceeds from issuance of common shares					$ 528,600,000	$ 378,500,000	$ 755,100,000