Stock-based compensation (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Deferred Compensation Arrangement with Individual, Share-based Payments
Allocated Share-based Compensation Expense	$ 6.1		$ 4.6	$ 15.2	$ 11.9
2010 Employee Stock Purchase Plan (ESPP)
Deferred Compensation Arrangement with Individual, Share-based Payments
Employee stock purchase plan shares issued during period	79,300	125,101
Physician Segment [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments
Allocated Share-based Compensation Expense	$ 0.1				$ 0.4