Share Incentive Plans, Stock Option Activity (Details) - 2006 and 2015 Plans [Member] - Stock Options [Member] - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Incentive Shares [Roll Forward]
Outstanding, beginning balance (in shares)	659,574	763,672	763,672
Granted (in shares)	155,780		124,100
Exercised (in shares)	(110,382)		(209,943)
Forfeited (in shares)	(7,376)		(18,255)
Outstanding, ending balance (in shares)	697,596		659,574
Weighted Average Exercise Price [Abstract]
Outstanding, beginning balance (in dollars per share)	$ 9.00	$ 5.73	$ 5.73
Granted (in dollars per share)	47.58		21.27
Exercised (in dollars per share)	5.78		3.97
Forfeited (in dollars per share)	33.49		13.46
Outstanding, ending balance (in dollars per share)	17.87		9.00
Weighted Average Grant Date Fair Value [Abstract]
Outstanding, beginning balance (in dollars per share)	3.34	$ 2.36	2.36
Granted (in dollars per share)	15.63		6.33
Exercised (in dollars per share)	2.60		1.47
Forfeited (in dollars per share)	10.73		3.90
Outstanding, ending balance (in dollars per share)	$ 6.13		$ 3.34
Intrinsic value of options exercised	$ 4.6	$ 5.2

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.