Subsequent Events (Details) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
		Jan. 21, 2016 USD ($)	Jan. 13, 2016 USD ($)	Feb. 11, 2016 USD ($)	Jan. 31, 2016 shares	Dec. 31, 2015 USD ($) building $ / shares shares	Dec. 31, 2014 USD ($) property $ / shares shares	Dec. 31, 2013 USD ($)	Dec. 31, 2011 $ / shares
Subsequent Event [Line Items]
Dispositions Sales Price	[1]					$ 309,200	$ 399,500	$ 56,900
Number of real estate properties						2	5
Payments of Ordinary Dividends, Common Stock						$ 126,839	$ 118,463	111,517
Common Stock, Dividends, Per Share, Declared | $ / shares						$ 1.40	$ 1.40		$ 1.40
Dividends, Preferred Stock						$ 13,250	$ 13,250	$ 13,250
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares						212,468
Subsequent Event [Member]
Subsequent Event [Line Items]
Dispositions Sales Price		$ 262,300	$ 4,500
Payments of Ordinary Dividends, Common Stock				$ 33,400
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares					328,731
Von Karman, Irvine, California [Member]
Subsequent Event [Line Items]
Dispositions Sales Price	[2],[3]					$ 26,000	$ 26,000
Market Measure-Based Restricted Stock Units (RSUs) [Member]
Subsequent Event [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares						127,657	119,098
Market Measure-Based Restricted Stock Units (RSUs) [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares					185,032

[1] Represents gross sales price before the impact of broker commissions and closing costs.

[2] Represents gross sales price before the impact of commissions and closing costs.

[3] This transaction resulted in a gain on sale of $17.3 million.