Income Taxes and Tax Credit Transactions (Textual) (Details)		1 Months Ended	12 Months Ended
	Dec. 30, 2008 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 27, 2010 USD ($)	Nov. 17, 2008 ft²
Tax Credit Carryforward [Line Items]
Accruals for tax uncertainties			$ 0	$ 0
Deferred income tax benefit			628,000	0	$ 0
Tax credit transaction income			$ 0	0	(19,955,000)
Historic Tax Credit Transaction [Member]
Tax Credit Carryforward [Line Items]
Area Of IRS Campus Building | ft²							862,692
Percentage of building leased to IRS							100.00%
Purchase price for noncontrolling interest obligation put option		$ 3,200,000
Rate of return on noncontrolling interest		2.00%
Percentage of tax credit recapture		20.00%
Tax credit transaction income		$ 11,900,000
Other income (expense)		$ (500,000)
Recognition of cash received as revenue net of allocated expenses on or after September 2011, period			over the five year credit recapture period as defined in the Internal Revenue Code
Recognition of cash received as revenue net of allocated expenses on or after September 2011, period in years		5 years
Accretion of noncontrolling interest liability				0	1,100,000
Historic Tax Credit Transaction [Member] | U S B [Member]
Tax Credit Carryforward [Line Items]
Agreed contribution in project cost						$ 64,100,000
USB contributions			$ 0	0
New Markets Tax Credit Transaction [Member]
Tax Credit Carryforward [Line Items]
Percentage of tax credit recapture			100.00%
Tax credit transaction income	$ 8,000,000				$ 8,100,000
Recognition of cash received as revenue net of allocated expenses on or after September 2011, period in years			7 years
Percentage of qualified investments	39.00%
New Markets Tax Credit Transaction [Member] | U S B [Member]
Tax Credit Carryforward [Line Items]
USB contributions	$ 13,300,000
Other Assets [Member]
Tax Credit Carryforward [Line Items]
Net deferred tax assets			$ 600,000
Other Assets [Member] | Historic Tax Credit Transaction [Member]
Tax Credit Carryforward [Line Items]
Deferred Costs			0	0
Other Assets [Member] | New Markets Tax Credit Transaction [Member]
Tax Credit Carryforward [Line Items]
Deferred Costs			$ 0	$ 0
Other Liabilities [Member] | Historic Tax Credit Transaction [Member]
Tax Credit Carryforward [Line Items]
Preferred return		$ 1,000,000