Equity Incentive Awards - Stock option activity (Details) - $ / shares		12 Months Ended
	May 02, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares Subject To Options
Beginning balance (in shares)		433,150	865,600	1,085,600
Options issued (in shares)	6,930		7,550
Options exercised (in shares)		(220,000)	(440,000)	(220,000)
Ending balance (in shares)		220,080	433,150	865,600	1,085,600
Exercisable at December 31 (in shares)		212,966
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Beginning balance (in usd per share)		$ 21.44	$ 23.12	$ 21.95
Options issued (in usd per share)	$ 81.15	11.00	10.70
Options exercised (in usd per share)		22.05	25.66	17.35
Ending balance (in usd per share)		18.32	$ 21.44	$ 23.12	$ 21.95
Exercisable at December 31 (in usd per share)		$ 20.84
Weighted Average Outstanding Contractual Life (in years)		1 year 6 months	1 year 8 months	1 year 7 months	2 years 1 month 1 day
Exercisable at December 31 (in years)		1 year 5 months