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) | |||||