Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and Cash Equivalents, Fair Value Disclosure	$ 61,661	$ 46,600
Available-for-sale Securities, Fair Value Disclosure	249,127	299,904
Investments, Fair Value Disclosure	310,788	346,504
Accrued Liabilities, Fair Value Disclosure		18,912
Liabilities, Fair Value Disclosure		18,912
Fair Value, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and Cash Equivalents, Fair Value Disclosure	61,661	45,093
Available-for-sale Securities, Fair Value Disclosure	222,148	227,805
Investments, Fair Value Disclosure	283,809	272,898
Accrued Liabilities, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure		0
Fair Value, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and Cash Equivalents, Fair Value Disclosure	0	1,507
Available-for-sale Securities, Fair Value Disclosure	26,979	72,099
Investments, Fair Value Disclosure	26,979	73,606
Accrued Liabilities, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure		0
Fair Value, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Available-for-sale Securities, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	$ 0	0
Accrued Liabilities, Fair Value Disclosure		18,912
Liabilities, Fair Value Disclosure		$ 18,912