Segment Information - Significant Customers (Details) - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues, Net | Walgreen Co.
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)	17.00%	19.00%	20.00%
Revenues, Net | Accredo/Curascript
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)	14.00%	15.00%	15.00%
Revenues, Net | CVS/Caremark
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)	10.00%	19.00%	17.00%
Accounts Receivable | Walgreen Co.
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)	20.00%	15.00%
Accounts Receivable | Accredo/Curascript
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)	12.00%	10.00%
Accounts Receivable | CVS/Caremark
Concentration Risk [Line Items]
Concentration risk percentage (less than for CVS/Caremark 2017 revenue)		17.00%