Debt and Credit Facilities - Margin for Borrowings under Revolving Credit Facility (Details) - Revolving Credit Facility $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Minimum | III
Line of Credit Facility [Line Items]
Availability of borrowing	$ 24
Minimum | II
Line of Credit Facility [Line Items]
Availability of borrowing	12
Maximum | II
Line of Credit Facility [Line Items]
Availability of borrowing	24
Maximum | I
Line of Credit Facility [Line Items]
Availability of borrowing	$ 12
Base Rate Loans | III
Line of Credit Facility [Line Items]
Basis spread (as a percent)	0.50%
Base Rate Loans | II
Line of Credit Facility [Line Items]
Basis spread (as a percent)	0.75%
Base Rate Loans | I
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.00%
LIBOR Revolver Loans | III
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.50%
LIBOR Revolver Loans | II
Line of Credit Facility [Line Items]
Basis spread (as a percent)	1.75%
LIBOR Revolver Loans | I
Line of Credit Facility [Line Items]
Basis spread (as a percent)	2.00%