Acquired Intangible Assets, Net (Schedule of Estimated Future Amortization Expense of Acquired Intangible Assets) (Details) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|---|
| Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] | |||
| 2019 | $ 20,472 | ||
| 2020 | 18,267 | ||
| 2021 | 17,470 | ||
| 2022 | 15,064 | ||
| 2023 | 13,669 | ||
| Thereafter | 69,262 | ||
| Total | $ 145,050 | $ 154,204 | $ 150,554 |