Lease Commitments - Narrative (Details) ft² in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	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 | $	$ 4.9	$ 5.1	$ 3.9
Long-term Purchase Commitment [Line Items]
Space under lease (sq ft)					58	132
Sublease
Long-term Purchase Commitment [Line Items]
Space under lease (sq ft)				3