Commitments and Contingencies - Letters of Credit and Guarantees (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Other commitments
Letters of credit and bank guarantees outstanding	$ 26,300,000	$ 28,700,000
Multiple Letters of Credit [Member] | Letter of Credit
Other commitments
Maximum borrowing capacity under letters of credit, excluding agreement with unspecified availability	80,000,000.0
Remaining borrowing capacity, under letters of credit with borrowing limits	$ 55,900,000