Debt Debt outstanding (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Credit spread		1.40%
All-in rate		2.66%
Secured		$ 288,355	$ 272,613
Unsecured notes		250,000	250,000
Unsecured term loan		150,000	150,000
Total debt		$ 824,355	$ 749,113
Weighted average interest rate		3.40%	3.70%
Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Secured		$ 222,419	$ 223,934
Secured Debt Weighted Average Interest Rate		4.80%	4.80%
Unsecured notes		$ 250,000	$ 250,000
Total debt		$ 472,419	$ 473,934
Weighted average interest rate		4.60%	4.60%
Fixed Rate Debt Unsecured Notes [Member] [Domain]
Debt Instrument [Line Items]
Unsecured Debt Weighted Average Interest Rate		4.40%	4.40%
Variable Rate Debt Hedged [Member]
Debt Instrument [Line Items]
Secured	[1]	$ 125,000
Secured Debt Weighted Average Interest Rate	[1]	2.70%
Unsecured term loan	[2],[3]		$ 125,000
Unsecured Debt Weighted Average Interest Rate	[2],[3]		2.70%
Total debt		$ 150,000	$ 125,000
Weighted average interest rate		2.50%	2.70%
Fixed Rate Debt Hedged [Member]
Debt Instrument [Line Items]
Fixed derivative rate		1.26%
Unsecured term loan	[2],[3]	$ 25,000
Unsecured Debt Weighted Average Interest Rate	[2],[3]	1.60%
Variable Rate Debt Unhedged [Member]
Debt Instrument [Line Items]
Secured		$ 65,936	$ 48,679
Secured Debt Weighted Average Interest Rate		1.50%	1.50%
Total debt		$ 201,936	$ 150,179
Weighted average interest rate		1.50%	1.50%
Variable Rate Debt Unhedged Revolver [Member]
Debt Instrument [Line Items]
Unsecured term loan		$ 136,000	$ 76,500
Unsecured Debt Weighted Average Interest Rate		1.50%	1.50%
Variable Rate Debt Unhedged Term Loan [Member]
Debt Instrument [Line Items]
Unsecured term loan			$ 25,000
Unsecured Debt Weighted Average Interest Rate		0.00%	1.60%
2018 derivative [Member]
Debt Instrument [Line Items]
Fixed derivative rate		1.75%
All-in rate		3.15%
2015 Derivative [Member]
Debt Instrument [Line Items]
Fixed derivative rate		1.42%
All-in rate		2.82%
2013 derivative [Domain]
Debt Instrument [Line Items]
Fixed derivative rate		1.55%
All-in rate		2.95%

[1] The Company entered into a forward starting swap in January 2015 fixing the rate beginning January 2016 at a rate of 1.42% per annum plus the credit spread which was 1.40% per annum as of June 30, 2015, or an all-in rate of 2.82% per annum until the loan matures in January 2020.

[2] The Company entered into a forward starting swap in December 2011 fixing the rate beginning in June 2013 until June 2016 at a rate of 1.26% per annum plus the credit spread which was 1.40% per annum as of June 30, 2015, or an all-in rate of 2.66% per annum. Additionally, the Company entered into a forward starting swap in April 2013 fixing the rate beginning June 2016 at a rate of 1.55% per annum plus the credit spread which was 1.40% per annum as of June 30, 2015, or an all-in rate of 2.95% per annum until January 2018.

[3] The Company entered into a forward starting swap in January 2015 fixing the rate beginning January 2018 at a rate of 1.75% per annum plus the credit spread which was 1.40% per annum as of June 30, 2015, or an all-in rate of 3.15% per annum until the loan matures in January 2020.