Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions |
Jun. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Senior Notes Due 2026 | Senior Notes | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Fair value of senior notes | $ 389.0 | $ 407.5 |