Non-Controlling Interests, Preferred Shares, Common Shares and Common Shares in Treasury - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 31, 2014	May 30, 2014
Schedule Of Equity [Line Items]
Operating partnership units outstanding	1,412,366	369,176
Redemption of preferred stock	$ 55,000,000	$ 150,000,000	$ 170,000,000
Write-off of preferred share original issuance costs	1,943,000	5,246,000	5,804,000
Common shares, par value	$ 0.10	$ 0.10
Preferred Shares
Schedule Of Equity [Line Items]
Fees paid for issuance of shares		400,000	800,000
Common Shares
Schedule Of Equity [Line Items]
Fees paid for issuance of shares	200,000	700,000	1,100,000
Class H cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Redemption of preferred stock				55,000,000
Cumulative redeemable preferred shares liquidation value		$ 500		$ 500.00
Cumulative redeemable preferred shares total redemption price per share				$ 504.6094
Redeemable preferred stock dividend per share prorated to redemption date				$ 4.6094
Write-off of preferred share original issuance costs				$ 1,900,000
Preferred Shares, without par value	750,000	750,000
Class H Depositary Share [Member]
Schedule Of Equity [Line Items]
Cumulative redeemable preferred shares liquidation value					$ 25.00
Cumulative redeemable preferred shares total redemption price per share					$ 25.2305
Redeemable preferred stock dividend per share prorated to redemption date					$ 0.2305
Class J depositary share [Member]
Schedule Of Equity [Line Items]
Preferred share, stated value	$ 500
Redemption date restrictions for depositary shares	Aug. 01, 2017
Class K depositary share [Member]
Schedule Of Equity [Line Items]
Preferred share, stated value	$ 500
Redemption date restrictions for depositary shares	Apr. 09, 2018
Class A cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class B cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class C cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class D cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class E cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class F cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class G cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class I cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Class J cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Cumulative redeemable preferred shares liquidation value	$ 500	$ 500
Preferred Shares, without par value	750,000	750,000
Class K cumulative redeemable preferred shares [Member]
Schedule Of Equity [Line Items]
Cumulative redeemable preferred shares liquidation value	$ 500	$ 500
Preferred Shares, without par value	750,000	750,000
Non-Cumulative Preferred Shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	750,000
Cumulative Voting Preferred Shares [Member]
Schedule Of Equity [Line Items]
Preferred Shares, without par value	2,000,000
Chicago, IL [Member]
Schedule Of Equity [Line Items]
Operating Partnership Units Issued	1,000,000