Retirement Plans - 401(k) Retirement Savings Plan (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
Retirement Benefits [Abstract] | |||
Employer contributions, net of forfeitures | $ 4,486 | $ 5,010 | $ 1,618 |