Debt Obligations (Details Textuals 1) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2011 Research Office Center [Member]	Sep. 30, 2011 Concord Airport Plaza [Member]	Dec. 31, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 15, 2011 Unsecured Debt [Member]	Dec. 31, 2010 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member] $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]		Dec. 31, 2010 Unsecured Debt [Member] $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount								$ 600,000,000		$ 345,000,000
Debt instrument stated interest rate						3.875%				3.875%
Unamortized fixed-rate debt premium				300,000	200,000
Senior unsecured notes discounts							5,177,000		2,763,000
Proceeds from Issuance of First Mortgage Bond	0	256,104,000	149,800,000
Debt obligations (Additional) (Textuals)
Repurchased outstanding unsecured notes	106,215,000
Long-term Debt, Gross						59,800,000	1,888,111,000		1,722,326,000	0		59,835,000
Less: unamortized exchangeable debt discount		900,000					0		906,000
Debt Instrument, Interest Rate, Effective Percentage	5.50%	5.50%								5.50%	[1]
Contractual coupon interest recognized on exchangeable debt	1,900,000	3,200,000
Interest on amortization of debt discount	900,000	1,600,000
Debt discount write-offs resulting from debt repurchases		2,000,000
Debt Instrument, Unamortized Premium				$ 300,000	$ 200,000

[1] On October 20, 2011, holders representing $59.5 million of the outstanding Exchangeable Notes exercised their right to cause the Company to redeem their notes at par plus accrued and unpaid interest leaving an outstanding balance of $0.4 million. On December 8, 2011, the Company redeemed the remaining balance of the Exchangeable Notes pursuant to its right under the indenture agreement (see related discussion below).