Share Based Compensation (Narrative) (Detail) (USD $)	10 Months Ended	12 Months Ended
	Feb. 27, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 23, 2012	Apr. 09, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate compensation expense		$ 800,000	$ 800,000	$ 500,000
Compensation costs related to share based awards		100,000	100,000	100,000
Aggregate fair value of restricted shares		4,300,000	4,100,000	6,200,000
Unrecognized compensation cost related to unvested share based compensation		3,700,000
Weighted average period		9 months 18 days
Fair value of shares vested		6,000,000	5,400,000	7,500,000
Number of days for acquisition of RSUs		20 days
Aggregate fair value of awards		500,000	600,000	400,000
Number of share issued		345,344	341,710	525,822
Unrecognized compensation expense		300,000
Grants in period (in shares)		5,000
Number of option exercised		5,000
Restricted Shares Awarded to Non-Employee Trustees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate compensation expense		8,500,000	7,300,000	11,100,000
Accrued amortization relating to employee separation		1,500,000	700,000	2,100,000
2003 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total remaining number of common shares to be issued		1,454,626
Time Based Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate compensation expense		4,900,000	4,300,000	6,000,000
Number of awards granted		225,978	222,664	425,462
Common share per share on the date of grant		$ 19.23	$ 18.29	$ 14.57
Accelerated amortization relating to employee separation		800,000	500,000
Time Based Vesting [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vest over period		1 year
Time Based Vesting [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vest over period		3 years
Restricted Share Unit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate compensation expense		2,800,000	2,300,000	4,500,000
Number of awards granted	112,898	127,353			80,744	134,761
Accelerated amortization relating to employee separation		700,000	200,000
Aggregate fair value of awards		2,200,000	2,000,000	4,000,000
Aggregate fair values of RSU awards per share		$ 17.56	$ 17.40	$ 18.41
Future compensation expense		1,600,000
Service Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate fair value of awards				29,000
Number of share issued				1,875
Non Employee Trustee Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period of unrecognized compensation expense		73 days
Fair value of shares granted to non-employee trustees		$ 900,000	$ 500,000	$ 100,000