Basis and Summary of Significant Accounting Policies - Investments (Details) $ in Thousands, £ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Impairment of Investment	$ (20,098)	$ 0	$ (20,098)	$ 0
Variable Interest Entity, Not Primary Beneficiary
Significant Accounting Policies [Line Items]
Held-to-maturity securities					£ 15.1	$ 20,400
Impairment of Investment	$ (20,100)