Debt (Details Textual) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended							6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Minimum [Member]	Jun. 30, 2014 Maximum [Member]	Jun. 30, 2014 credit facility [Member]	Jun. 30, 2014 Revolving facility [Member]	Jun. 30, 2014 Term Loan [Member]	Aug. 13, 2012 Term Loan [Member]	Jun. 30, 2014 Term Loan [Member]	Jun. 30, 2014 Accumulated Other Comprehensive Income (Loss) [Member]	Jun. 30, 2013 Accumulated Other Comprehensive Income (Loss) [Member]
Line of Credit Facility [Line Items]
Senior unsecured revolving credit facility								$ 300,000,000	$ 200,000,000		$ 100,000,000	$ 100,000,000
Credit facility maturity date			Jul. 31, 2016
Credit facility maturity extension option			1 year
Line of Credit Facility, Maximum Borrowing Capacity	600,000,000		600,000,000
Credit facility borrowings LIBOR rate plus						1.75%	2.50%
Annual rate of unused commitment fee						0.25%	0.35%
Outstanding borrowings under the credit facility	36,000,000.0		36,000,000.0		0
Unused commitment fees	200,000	200,000	300,000	400,000
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	100,000,000		100,000,000
Debt Instrument, Interest Rate, Stated Percentage										2.55%
Derivative Asset, Fair Value	500,000		500,000
Unrealized gain (loss) on derivative instruments	(462,000)	1,695,000	(578,000)	1,800,000									(578,000)	1,800,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	100,000	100,000	300,000	300,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred			500,000
Estimated fair value of debt	$ 453,500,000		$ 453,500,000		$ 460,900,000