Long-Term Debt - Components of Long-Term Debt (Parenthetical) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2010	Dec. 31, 2009
TDC | Senior Secured Notes, 8.50%
Debt Instrument [Line Items]
Long-term debt, face amount	$ 25.0	$ 25.0
SDTS | Senior Secured Term Loan
Debt Instrument [Line Items]
Long-term debt, face amount	200.0
SDTS | Senior Secured Notes, 3.86%
Debt Instrument [Line Items]
Long-term debt, face amount	400.0
SDTS | Senior Secured Notes, 3.86% Maturing in 2026
Debt Instrument [Line Items]
Long-term debt, face amount	100.0
SDTS | Senior Secured Notes, 7.25%
Debt Instrument [Line Items]
Long-term debt, face amount	53.5		$ 53.5
SDTS | Senior Secured Notes, 6.47%
Debt Instrument [Line Items]
Long-term debt, face amount	$ 110.0	$ 110.0