Long-Term Incentive Plan (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2008
Long-Term Incentive Plan
Number of common shares reserved for issuance under LTIP (in shares)	12,050,000
Number of shares available for future issuance (in shares)	6,537,837
Amortization related to shares issued to design and construction employees	$ 474	$ 393	$ 302
Vesting of restricted stock presented on statement of equity	6,695	5,251	3,768
Non-vested stock grants, number of shares
Outstanding at the beginning of the period (in shares)	2,009,412	1,539,992	1,407,152
Granted (in shares)	691,182	975,711	647,171
Vested (in shares)	(799,845)	(497,199)	(513,095)
Forfeited (in shares)	(17,453)	(9,092)	(1,236)
Outstanding at the end of the period (in shares)	1,883,296	2,009,412	1,539,992
Non-vested stock grants, weighted average price
Outstanding at the beginning of the period (in dollars per share)	$ 10.23	$ 9.11	$ 8.55
Granted (in dollars per share)	$ 13.48	$ 11.82	$ 9.51
Vested (in dollars per share)	$ 10.61	$ 9.89	$ 8.08
Forfeited (in dollars per share)	$ 11.90	$ 10.89	$ 9.38
Outstanding at the end of the period (in dollars per share)	$ 11.24	$ 10.23	$ 9.11
Restricted Shares and Performance awards
Long-Term Incentive Plan
Compensation Expense, including forfeitures	9,063	7,189	5,139
Net-settle adjustment	(2,842)	(2,331)	(1,673)
Amortization related to shares issued to design and construction employees	474	393	302
Vesting of restricted stock presented on statement of equity	$ 6,695	$ 5,251	$ 3,768
Restricted Shares and Performance awards | Minimum
Long-Term Incentive Plan
Vesting period	3 years
Restricted Shares and Performance awards | Maximum
Long-Term Incentive Plan
Vesting period	5 years
Stock Options | Robert A. Alter
Stock options
Number of nonqualified stock options approved by the compensation committee of the Company's board of directors (in shares)				200,000
Exercise price of options vested (in dollars per share)				$ 17.71
Deferred shares, share purchase rights, or share appreciation rights issued or outstanding under the LTIP
Non-vested stock grants, number of shares
Outstanding at the end of the period (in shares)	0