Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, at amortized cost	$ 315,329	$ 347,615	$ 334,291
Operating leases (less than)	2,600
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, at amortized cost	1,120	$ 1,120	$ 1,120
Accounting Standards Update 2016-09
Schedule of Available-for-sale Securities [Line Items]
Effect of change on net income	$ 112
Effect of change on earnings per share (less than) (in usd per share)	$ 0.01
Fair Value, Measurements, Recurring | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, at amortized cost	$ 1,500