Debt - Credit Facility (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
U.S revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum amount	$ 175,000,000.0
Borrowing availability	175,000,000
Outstanding letters of credit issued	44,800,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Letter of credit sub-facility amount	106,900,000
Outstanding borrowings	23,300,000
Long-term Debt [Member] | U.S revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility face amount	175,000,000.0
Long-term Debt [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Letter of credit sub-facility amount	110,000,000
Long-term Debt [Member] | U.S revolving credit facility, accordion [Member]
Line of Credit Facility [Line Items]
Credit facility face amount	50,000,000
Long-term Debt [Member] | Iceland revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum amount	50,000,000
Borrowing availability	50,000,000
Outstanding letters of credit issued	0
Borrowing availability, net of outstanding letters of credit and borrowings	50,000,000
Short-term Debt [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings	23,300,000	$ 0
Short-term Debt [Member] | Iceland revolving credit facility [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings	$ 0