Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized share-based compensation expense	$ 103.7		$ 103.7
Recognized weighted average period			3 years 2 months
Shares of common stock exercised (in shares)			33,854
Intrinsic value of common stock exercised			$ 0.5
Warrants outstanding (in shares)	10,000		10,000
Warrants exercised during period (in shares)			83,013
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period (in shares)			43,056
2013 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant (in shares)	1,258,010		1,258,010
Additional shares reserved for future issuance (in shares)			6,621,170
Public Stock Offering [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issued (in shares)		3,731,344
Price per share (usd per share)		$ 33.50
Gross proceeds		$ 125.0
Proceeds, net of underwriting commissions		117.5
Underwriting commissions		$ 7.5
Underwritten Public Offering [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issued (in shares)	3,910,000
Price per share (usd per share)	$ 29.50		$ 29.50
Gross proceeds	$ 115.3
Proceeds, net of underwriting commissions	109.8
Underwriting commissions	$ 5.5