Non-Controlling Interests, Preferred Shares, Common Shares and Common Shares in Treasury - Preferred Shares Outstanding (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Preferred Shares Outstanding [Line Items]
Cumulative redeemable preferred shares	$ 350,000	$ 405,000
Class H cumulative redeemable preferred shares [Member]
Preferred Shares Outstanding [Line Items]
Cumulative redeemable preferred shares	0	55,000
Class J cumulative redeemable preferred shares [Member]
Preferred Shares Outstanding [Line Items]
Cumulative redeemable preferred shares	200,000	200,000
Class K cumulative redeemable preferred shares [Member]
Preferred Shares Outstanding [Line Items]
Cumulative redeemable preferred shares	$ 150,000	$ 150,000