Short-Term Debt - Keystone Acquisition - Additional Information (Details) - Revolving Credit Facility - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 1,762,000,000	$ 1,766,000,000
Keystone Clearwater Solutions
Short-term Debt [Line Items]
Maximum borrowing capacity	12,000,000
Available credit facility capacity	7,000,000
Line of credit facility, outstanding borrowings	$ 0
Derivative description of variable rate basis	one month or one day LIBOR
Line of credit facility, borrowing base, description	The borrowing base under the facility allows for financing up to the greater of the note or 80% of eligible accounts receivable.
Percentage of eligible accounts receivable	80.00%
Keystone Clearwater Solutions | London Interbank Offered Rate (LIBOR)
Short-term Debt [Line Items]
Debt, basis spread on variable rate	2.75%