Summary of Significant Accounting Policies - Intangible Assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets
Amortization expense recognized	$ 7.5	$ 6.8	$ 6.1
Amortization expense expected to be recognized, 2019	7.2
Amortization expense expected to be recognized, 2020	7.0
Amortization expense expected to be recognized, 2021	7.0
Amortization expense expected to be recognized, 2022	5.9
Amortization expense expected to be recognized, 2023	5.5
Amortization expense expected to be recognized, thereafter	$ 22.8
Minimum
Finite-Lived Intangible Assets
Estimated useful lives	5 years
Maximum
Finite-Lived Intangible Assets
Estimated useful lives	20 years