Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) plan $ / shares	Dec. 31, 2016 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of compensation plans | plan	2
Award vesting period (in years)	4 years
ESPP, market price discount percentage	5.00%
Available-for-Sale	$ 300,200	$ 346,996
Available-For-Sale Securities, at Amortized Cost	301,876	347,615
Operating leases, future minimum payments due, future minimum sublease rentals	2,200
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, effect of change on net income	$ 148
New accounting pronouncement, effect of change on basic earnings per share | $ / shares	$ 0.01
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings	$ (1,600)
Adjustments to additional paid in capital, income tax rate reduction	1,600
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings	330
Fair Value, Measurements, Recurring [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available-for-Sale	300,200	346,996
Equity Securities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available-for-Sale	1,561	1,382
Available-For-Sale Securities, at Amortized Cost	1,120	1,120
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available-for-Sale	$ 1,561	$ 1,382
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible asset, useful life (in years)	10 years
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible asset, useful life (in years)	15 years
Building and Building Improvements [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Building and Building Improvements [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	40 years
Furniture and Fixtures [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	7 years
Furniture and Fixtures [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Computer Equipment [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Computer Equipment [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life	7 years