Issuance and Redemption of Preferred Stock (Detail) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended		9 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2014	Dec. 31, 2006	Mar. 15, 2012	Apr. 30, 2012	Feb. 29, 2012	Sep. 30, 2012	Mar. 31, 2012
Issuance and redemption of preferred stock
Excess of redemption value over carrying value	$ 6,015,000		$ 3,696,000
Underwriting discounts and other offering costs	22,827,000	55,359,000	13,773,000
Net proceeds from preferred stock offering			395,377,000
Redemption of preferred stock	220,000,000		127,500,000
Class E Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Shares issued in stock offerings, net of offering costs (in shares)					8,800,000
Preferred stock, dividend rate (as a percent)					6.75%
Offering price per share (in dollars per share)					$ 25.00
Excess of redemption value over carrying value				6,000,000
Redemption amount (in dollars per share)				$ 25.00
Preferred stock redeemed (in shares)				8,800,000
Class F Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Shares issued in stock offerings, net of offering costs (in shares)							1,400,000	14,950,000
Preferred stock, dividend rate (as a percent)								6.625%
Offering price per share (in dollars per share)							$ 25.2863	$ 25.00
Preferred stock, purchased by underwriters (in shares)								1,950,000
Underwriting discounts and other offering costs									13,800,000
Net proceeds from preferred stock offering									395,400,000
Redemption of preferred stock									127,500,000
Redemption amount (in dollars per share)	$ 25.00
Monthly distributions payable (in dollars per share)	$ 0.138021
Dividend paid per initial preferred share of stock (in dollars per share)						$ 0.1702257
Period when initial dividend payments were made						37 days
Class D Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Preferred stock, dividend rate (as a percent)									7.375%
Redemption amount (in dollars per share)										$ 25.00
Preferred stock redeemed (in shares)										5,100,000
Original issuance costs			$ 3,700,000