Equity Incentive Awards - Stock option activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Payment Arrangement [Abstract]
Options forfeited (in shares)	0	0	0
Options expired (in shares)	0	0	0
Shares Subject To Options
Beginning balance (in shares)	47,100	440,160	866,300
Options issued (in shares)		12,540	13,860
Options exercised (in shares)	(5,600)	(405,600)	(440,000)
Ending balance (in shares)	41,500	47,100	440,160	866,300
Exercisable at December 31, 2019	37,782
Weighted Average Exercise Price Per Share
Beginning balance (in usd per share)	$ 36.95	$ 11.36	$ 10.72
Options issued (in usd per share)		44.83	40.58
Options exercised (in usd per share)	9.43	9.43	11.02
Ending balance (in usd per share)	40.65	$ 36.95	$ 11.36	$ 10.72
Exercisable at December 31, 2019	$ 40.24
Weighted Average Outstanding Contractual Life (in years)
Balance outstanding, Weighted Average Outstanding Contractual Life (in years)	7 years 3 months 18 days	7 years 3 months 18 days	1 year 7 months 6 days	1 year 8 months 12 days
Exercisable at end of the period, Weighted Average Outstanding Contractual Life (in years)	7 years 2 months 12 days
Average Intrinsic Value (in millions)
Opening Balance, Average Intrinsic Value	$ 500,000	$ 14,600,000	$ 22,000,000.0
Options exercised, Average Intrinsic Value	0.2	16,900,000	14,500,000
Closing Balance, Average Intrinsic Value	1,200,000	$ 500,000	$ 14,600,000	$ 22,000,000.0
Exercisable at December 31, 2019	$ 1,100,000