Summary of Significant Accounting Policies - Goodwill and Indefinite-lived Intangibles Impairment (Details 9) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impairment of Long-Lived Assets and Long-lived Assets to be Disposed of
Goodwill, loss	$ 0	$ 0	$ 0
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 0	$ 0	$ 0