Fair Value Measurements and Disclosures (Details 1) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Liabilities
Secured Debt	[1]	$ 475,923	$ 546,292
Unsecured debt		2,181,382	1,783,121
Unsecured line of credit		0	140,000
Carrying Value
Liabilities
Secured Debt	[2]	475,923	546,292
Unsecured debt	[3]	2,181,382	1,783,121
Unsecured line of credit	[2],[4]	0	140,000
Carrying Value | Fair Value (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		189,000	513,700
Carrying Value | Unsecured Debt [Member] | Fair Value (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,992,300	1,269,400
Fair Value
Liabilities
Secured Debt	[2]	487,563	559,483
Unsecured debt	[3]	2,228,225	1,858,492
Unsecured line of credit	[2],[4]	0	145,051
Fair Value | Fair Value (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		189,000	536,300
Fair Value | Unsecured Debt [Member] | Fair Value (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		$ 2,039,200	$ 1,322,200

[1] Amounts reported include the amounts of unamortized debt premiums of $7.2 million and $10.3 million as of September 30, 2015 and December 31, 2014, respectively.

[2] Fair value calculated using Level II inputs, which are based on model-derived valuations in which significant inputs and significant value drivers are observable in active markets.

[3] Fair value calculated using Level I and Level II inputs. Level I inputs are based on quoted prices for identical instruments in active markets. The carrying value and fair value of the Level I instruments was $2.0 billion and $2.0 billion, respectively, as of September 30, 2015. The carrying value and fair value of the Level I instruments as of December 31, 2014, was $1.3 billion and $1.3 billion, respectively. The carrying value and fair value of the Level II instruments was $189.0 million and $189.0 million, respectively, as of September 30, 2015. The carrying value and fair value of the Level II instruments as of December 31, 2014, was $513.7 million and $536.3 million, respectively.

[4] There was no outstanding balance on the unsecured line of credit as of September 30, 2015.