Equity Incentive Awards - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options forfeited (in shares)	0	0	0
Options expired (in shares)	0	0	0
Shares Subject To Options
Beginning balance (in shares)	220,080	433,150	865,600
Options issued (in shares)	6,270	6,930	7,550
Options exercised (in shares)	(202,800)	(220,000)	(440,000)
Ending balance (in shares)	23,550	220,080	433,150	865,600
Exercisable at December 31, 2018	17,084
Weighted Average Exercise Price Per Share
Beginning balance (in usd per share)	$ 22.72	$ 21.44	$ 23.12
Options issued (in usd per share)	89.65	81.15	74.53
Options exercised (in usd per share)	18.85	22.05	25.66
Ending balance (in usd per share)	73.89	$ 22.72	$ 21.44	$ 23.12
Exercisable at December 31, 2018	$ 68.94
Weighted Average Outstanding Contractual Life (in years)
Balance outstanding, Weighted Average Outstanding Contractual Life (in years)	7 years 3 months 18 days	1 year 7 months 6 days	1 year 8 months 12 days	1 year 6 months
Exercisable at end of the period, Weighted Average Outstanding Contractual Life (in years)	6 years 8 months 12 days
Average Intrinsic Value (in millions)
Opening Balance, Average Intrinsic Value	$ 14.6	$ 22.0	$ 37.7
Options exercised, Average Intrinsic Value	16.9	14.5	18.7
Closing Balance, Average Intrinsic Value	0.5	$ 14.6	$ 22.0	$ 37.7
Exercisable at December 31, 2018	$ 0.5