Summary of business and significant accounting policies (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			6 Months Ended
	Aug. 30, 2019	Jul. 12, 2019	Jul. 31, 2019	Feb. 01, 2019	Jan. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities investment ownership percentage			4.00%
Marketable equity securities, at fair value			$ 81,130		$ 0
Equity method investment ownership percentage			22.00%
Equity method investments			$ 209		209
Cost method investment ownership percentage			1.00%
Non-marketable equity securities			$ 500		$ 500
Operating lease liabilities			39,614
Operating lease right of use asset			$ 36,716
Follow-on equity offering
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, shares (in shares)		7,762,500
Offering price per share (in usd per share)		$ 61.00
Net proceeds from follow on offering		$ 458,500
Underwriting discounts and commissions		14,100
Other offering expenses payable		$ 900
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Remaining operating lease terms			3 years
Lease renewal terms extension			3 years
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Remaining operating lease terms			11 years
Lease renewal terms extension			10 years
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities				$ 40,600
Operating lease right of use asset				$ 38,000
WageWorks, Inc | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Consideration paid for merger	$ 2,000,000