Lease Commitments - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft²	Feb. 29, 2016 ft²	Jan. 01, 2015 ft²	Jan. 31, 2014 ft²
Commitments and Contingencies Disclosure [Abstract]
Rent expense, excluding costs associated with the termination of noncancelable lease arrangements | $	$ 5.1	$ 3.9	$ 4.6
Long-term Purchase Commitment [Line Items]
Space under lease (sq ft)					58,000	132,000
Sublease
Long-term Purchase Commitment [Line Items]
Space under lease (sq ft)			0	3,000