Capital and Financing Transactions - Notes Payable (Details) (USD $)	3 Months Ended				12 Months Ended				0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012	Jun. 12, 2013	Feb. 10, 2012	Apr. 01, 2014	Oct. 01, 2012	Apr. 08, 2014	Oct. 10, 2012
Line of Credit Facility [Line Items]
Outstanding balance	$ 481,500,000				$ 481,500,000	$ 303,000,000
Loss on extinguishment of debt	2,066,000	0	339,000	0	2,405,000	0		0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit capacity after first modification														215,000,000
$10.0 Million Working Capital Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate	1.70%				1.70%
Outstanding balance	0				0
Unsecured Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity									120,000,000
Debt instrument variable rate basis									LIBOR
Basis spread on variable rate (in percent)									1.45%
Interest rate						2.50%	[1]
Outstanding balance						125,000,000	[1]
Term Loan Swap
Line of Credit Facility [Line Items]
Notional amount									120,000,000.0
Unsecured Term Loan $120 million
Line of Credit Facility [Line Items]
Interest rate									3.30%
Outstanding balance						120,000,000
Mortgage notes payable											120,000,000.0
Term Loan
Line of Credit Facility [Line Items]
Interest rate						3.30%	[2]
Outstanding balance						120,000,000	[2]
Unsecured Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate	1.60%				1.60%	2.00%
Outstanding balance	131,500,000				131,500,000	58,000,000
Mortgage notes payable											215,000,000.0
Unsecured Term Loan $125 million
Line of Credit Facility [Line Items]
Mortgage notes payable											125,000,000.0
Wells-Fargo | Unsecured Term Loan $120 million
Line of Credit Facility [Line Items]
Interest rate									3.30%
Wells-Fargo | Term Loan
Line of Credit Facility [Line Items]
Outstanding balance						120,000,000
Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)										2.50%
Minimum | Unsecured Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)									1.45%
Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)										3.50%
Maximum | Unsecured Term Loan
Line of Credit Facility [Line Items]
Accordion feature									250,000,000
Basis spread on variable rate (in percent)									2.00%
London Interbank Offered Rate (LIBOR) | $10.0 Million Working Capital Revolving Credit Facility
Line of Credit Facility [Line Items]
LIBOR rate period fixed									1.60%
Term loan facility swap terms									5 years
Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity											250,000,000.0
Term of extension option on debt instrument											1 year
Period end interest rate (in percent)		1.56%
Line of Credit | Unsecured Term Loan
Line of Credit Facility [Line Items]
Period end interest rate (in percent)												2.45%
Line of Credit | Term Loan
Line of Credit Facility [Line Items]
Period end interest rate (in percent)									3.30%
Line of Credit | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate during the period (in percent)											1.40%
Line of Credit | London Interbank Offered Rate (LIBOR) | Unsecured Term Loan
Line of Credit Facility [Line Items]
Interest rate during the period (in percent)												1.75%
Line of Credit | London Interbank Offered Rate (LIBOR) | Term Loan
Line of Credit Facility [Line Items]
Interest rate during the period (in percent)									1.70%
Line of Credit | London Interbank Offered Rate (LIBOR) | Minimum | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											1.40%
Line of Credit | London Interbank Offered Rate (LIBOR) | Minimum | Unsecured Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)												1.50%
Line of Credit | London Interbank Offered Rate (LIBOR) | Minimum | Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)									1.45%
Line of Credit | London Interbank Offered Rate (LIBOR) | Maximum | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											2.00%
Line of Credit | London Interbank Offered Rate (LIBOR) | Maximum | Unsecured Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)												2.25%
Line of Credit | London Interbank Offered Rate (LIBOR) | Maximum | Term Loan
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)									2.20%
Five-year Term Loan | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity											250,000,000.0
Term of debt instrument											5 years
Period end interest rate (in percent)											2.51%
Five-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Interest rate during the period (in percent)											1.35%
Five-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR) | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											1.35%
Five-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR) | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											1.90%
Seven-year Term Loan | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity											100,000,000.0
Term of debt instrument											7 years
Period end interest rate (in percent)											4.31%
Number of draw permitted on line of credit over 12-month period											2
Number of draw permitted period											12 months
Seven-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Interest rate during the period (in percent)											1.75%
Seven-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR) | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											1.75%
Seven-year Term Loan | Unsecured Debt | London Interbank Offered Rate (LIBOR) | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (in percent)											2.30%
Amended Working Capital Revolving Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity											10,000,000.0
Bank of America Center | First Mortgage
Line of Credit Facility [Line Items]
Repayments of debt													33,900,000
Mortgages | Bank of America Center
Line of Credit Facility [Line Items]
Loss on extinguishment of debt													$ 339,000

[1] Effective October 1, 2012, the Company executed two floating-to-fixed interest rate swaps associated with the Term Loan Facility totaling $125 million, locking LIBOR at 0.7% for five years. The loan bears interest at LIBOR plus the applicable spread which ranges between 150 to 225 basis points based on overall Company leverage. The current spread associated with the loan is 1.75% resulting in an all-in rate of 2.45%.

[2] Effective June 12, 2013, the Company entered into a new floating-to-fixed interest rate swap associated with a New Term Loan Facility totaling $120 million, locking LIBOR at 1.6% for five years. The loan bears interest at LIBOR plus the applicable spread which ranges between 145 to 220 basis points based on overall Company leverage. The current spread associated with the loan is 1.7% resulting in an all-in rate of 3.3%.